=============== Model reference =============== A model is the single, definitive source of data about your data. It contains the essential fields and behaviors of the data you're storing. Generally, each model maps to a single database table. The basics: * Each model is a Python class that subclasses ``django.db.models.Model``. * Each attribute of the model represents a database field. * Model metadata (non-field information) goes in an inner class named ``Meta``. * Metadata used for Django's admin site goes into an inner class named ``Admin``. * With all of this, Django gives you an automatically-generated database-access API, which is explained in the `Database API reference`_. A companion to this document is the `official repository of model examples`_. (In the Django source distribution, these examples are in the ``tests/modeltests`` directory.) .. _Database API reference: ../db-api/ .. _official repository of model examples: ../models/ Quick example ============= This example model defines a ``Person``, which has a ``first_name`` and ``last_name``:: from django.db import models class Person(models.Model): first_name = models.CharField(max_length=30) last_name = models.CharField(max_length=30) ``first_name`` and ``last_name`` are *fields* of the model. Each field is specified as a class attribute, and each attribute maps to a database column. The above ``Person`` model would create a database table like this:: CREATE TABLE myapp_person ( "id" serial NOT NULL PRIMARY KEY, "first_name" varchar(30) NOT NULL, "last_name" varchar(30) NOT NULL ); Some technical notes: * The name of the table, ``myapp_person``, is automatically derived from some model metadata but can be overridden. See `Table names`_ below. * An ``id`` field is added automatically, but this behavior can be overridden. See `Automatic primary key fields`_ below. * The ``CREATE TABLE`` SQL in this example is formatted using PostgreSQL syntax, but it's worth noting Django uses SQL tailored to the database backend specified in your `settings file`_. .. _settings file: ../settings/ Fields ====== The most important part of a model -- and the only required part of a model -- is the list of database fields it defines. Fields are specified by class attributes. Example:: class Musician(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) instrument = models.CharField(max_length=100) class Album(models.Model): artist = models.ForeignKey(Musician) name = models.CharField(max_length=100) release_date = models.DateField() num_stars = models.IntegerField() Field name restrictions ----------------------- Django places only two restrictions on model field names: 1. A field name cannot be a Python reserved word, because that would result in a Python syntax error. For example:: class Example(models.Model): pass = models.IntegerField() # 'pass' is a reserved word! 2. A field name cannot contain more than one underscore in a row, due to the way Django's query lookup syntax works. For example:: class Example(models.Model): foo__bar = models.IntegerField() # 'foo__bar' has two underscores! These limitations can be worked around, though, because your field name doesn't necessarily have to match your database column name. See `db_column`_ below. SQL reserved words, such as ``join``, ``where`` or ``select``, *are* allowed as model field names, because Django escapes all database table names and column names in every underlying SQL query. It uses the quoting syntax of your particular database engine. Field types ----------- Each field in your model should be an instance of the appropriate ``Field`` class. Django uses the field class types to determine a few things: * The database column type (e.g. ``INTEGER``, ``VARCHAR``). * The widget to use in Django's admin interface, if you care to use it (e.g. ``<input type="text">``, ``<select>``). * The minimal validation requirements, used in Django's admin and in manipulators. Here are all available field types: ``AutoField`` ~~~~~~~~~~~~~ An ``IntegerField`` that automatically increments according to available IDs. You usually won't need to use this directly; a primary key field will automatically be added to your model if you don't specify otherwise. See `Automatic primary key fields`_. ``BooleanField`` ~~~~~~~~~~~~~~~~ A true/false field. The admin represents this as a checkbox. ``CharField`` ~~~~~~~~~~~~~ A string field, for small- to large-sized strings. For large amounts of text, use ``TextField``. The admin represents this as an ``<input type="text">`` (a single-line input). ``CharField`` has an extra required argument, ``max_length``, the maximum length (in characters) of the field. The max_length is enforced at the database level and in Django's validation. Django veterans: Note that the argument is now called ``max_length`` to provide consistency throughout Django. There is full legacy support for the old ``maxlength`` argument, but ``max_length`` is preferred. ``CommaSeparatedIntegerField`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A field of integers separated by commas. As in ``CharField``, the ``max_length`` argument is required. ``DateField`` ~~~~~~~~~~~~~ A date field. Has a few extra optional arguments: ====================== =================================================== Argument Description ====================== =================================================== ``auto_now`` Automatically set the field to now every time the object is saved. Useful for "last-modified" timestamps. Note that the current date is *always* used; it's not just a default value that you can override. ``auto_now_add`` Automatically set the field to now when the object is first created. Useful for creation of timestamps. Note that the current date is *always* used; it's not just a default value that you can override. ====================== =================================================== The admin represents this as an ``<input type="text">`` with a JavaScript calendar, and a shortcut for "Today." The JavaScript calendar will always start the week on a Sunday. ``DateTimeField`` ~~~~~~~~~~~~~~~~~ A date and time field. Takes the same extra options as ``DateField``. The admin represents this as two ``<input type="text">`` fields, with JavaScript shortcuts. ``DecimalField`` ~~~~~~~~~~~~~~~~ **New in Django development version** A fixed-precision decimal number, represented in Python by a ``Decimal`` instance. Has two **required** arguments: ====================== =================================================== Argument Description ====================== =================================================== ``max_digits`` The maximum number of digits allowed in the number. ``decimal_places`` The number of decimal places to store with the number. ====================== =================================================== For example, to store numbers up to 999 with a resolution of 2 decimal places, you'd use:: models.DecimalField(..., max_digits=5, decimal_places=2) And to store numbers up to approximately one billion with a resolution of 10 decimal places:: models.DecimalField(..., max_digits=19, decimal_places=10) The admin represents this as an ``<input type="text">`` (a single-line input). ``EmailField`` ~~~~~~~~~~~~~~ A ``CharField`` that checks that the value is a valid e-mail address. In Django 0.96, this doesn't accept ``max_length``; its ``max_length`` is automatically set to 75. In the Django development version, ``max_length`` is set to 75 by default, but you can specify it to override default behavior. ``FileField`` ~~~~~~~~~~~~~ A file-upload field. Has one **required** argument: ====================== =================================================== Argument Description ====================== =================================================== ``upload_to`` A local filesystem path that will be appended to your ``MEDIA_ROOT`` setting to determine the output of the ``get_<fieldname>_url()`` helper function. ====================== =================================================== This path may contain `strftime formatting`_, which will be replaced by the date/time of the file upload (so that uploaded files don't fill up the given directory). The admin represents this field as an ``<input type="file">`` (a file-upload widget). Using a ``FileField`` or an ``ImageField`` (see below) in a model takes a few steps: 1. In your settings file, you'll need to define ``MEDIA_ROOT`` as the full path to a directory where you'd like Django to store uploaded files. (For performance, these files are not stored in the database.) Define ``MEDIA_URL`` as the base public URL of that directory. Make sure that this directory is writable by the Web server's user account. 2. Add the ``FileField`` or ``ImageField`` to your model, making sure to define the ``upload_to`` option to tell Django to which subdirectory of ``MEDIA_ROOT`` it should upload files. 3. All that will be stored in your database is a path to the file (relative to ``MEDIA_ROOT``). You'll most likely want to use the convenience ``get_<fieldname>_url`` function provided by Django. For example, if your ``ImageField`` is called ``mug_shot``, you can get the absolute URL to your image in a template with ``{{ object.get_mug_shot_url }}``. For example, say your ``MEDIA_ROOT`` is set to ``'/home/media'``, and ``upload_to`` is set to ``'photos/%Y/%m/%d'``. The ``'%Y/%m/%d'`` part of ``upload_to`` is strftime formatting; ``'%Y'`` is the four-digit year, ``'%m'`` is the two-digit month and ``'%d'`` is the two-digit day. If you upload a file on Jan. 15, 2007, it will be saved in the directory ``/home/media/photos/2007/01/15``. If you want to retrieve the upload file's on-disk filename, or a URL that refers to that file, or the file's size, you can use the ``get_FOO_filename()``, ``get_FOO_url()`` and ``get_FOO_size()`` methods. They are all documented here__. __ ../db-api/#get-foo-filename Note that whenever you deal with uploaded files, you should pay close attention to where you're uploading them and what type of files they are, to avoid security holes. *Validate all uploaded files* so that you're sure the files are what you think they are. For example, if you blindly let somebody upload files, without validation, to a directory that's within your Web server's document root, then somebody could upload a CGI or PHP script and execute that script by visiting its URL on your site. Don't allow that. .. _`strftime formatting`: http://docs.python.org/lib/module-time.html#l2h-1941 **New in development version:** By default, ``FileField`` instances are created as ``varchar(100)`` columns in your database. As with other fields, you can change the maximum length using the ``max_length`` argument. ``FilePathField`` ~~~~~~~~~~~~~~~~~ A field whose choices are limited to the filenames in a certain directory on the filesystem. Has three special arguments, of which the first is **required**: ====================== =================================================== Argument Description ====================== =================================================== ``path`` Required. The absolute filesystem path to a directory from which this ``FilePathField`` should get its choices. Example: ``"/home/images"``. ``match`` Optional. A regular expression, as a string, that ``FilePathField`` will use to filter filenames. Note that the regex will be applied to the base filename, not the full path. Example: ``"foo.*\.txt$"``, which will match a file called ``foo23.txt`` but not ``bar.txt`` or ``foo23.gif``. ``recursive`` Optional. Either ``True`` or ``False``. Default is ``False``. Specifies whether all subdirectories of ``path`` should be included. ====================== =================================================== Of course, these arguments can be used together. The one potential gotcha is that ``match`` applies to the base filename, not the full path. So, this example:: FilePathField(path="/home/images", match="foo.*", recursive=True) ...will match ``/home/images/foo.gif`` but not ``/home/images/foo/bar.gif`` because the ``match`` applies to the base filename (``foo.gif`` and ``bar.gif``). **New in development version:** By default, ``FilePathField`` instances are created as ``varchar(100)`` columns in your database. As with other fields, you can change the maximum length using the ``max_length`` argument. ``FloatField`` ~~~~~~~~~~~~~~ **Changed in Django development version** A floating-point number represented in Python by a ``float`` instance. The admin represents this as an ``<input type="text">`` (a single-line input). **NOTE:** The semantics of ``FloatField`` have changed in the Django development version. See the `Django 0.96 documentation`_ for the old behavior. .. _Django 0.96 documentation: http://www.djangoproject.com/documentation/0.96/model-api/#floatfield ``ImageField`` ~~~~~~~~~~~~~~ Like `FileField`_, but validates that the uploaded object is a valid image. Has two extra optional arguments, ``height_field`` and ``width_field``, which, if set, will be auto-populated with the height and width of the image each time a model instance is saved. In addition to the special ``get_FOO_*`` methods that are available for ``FileField``, an ``ImageField`` also has ``get_FOO_height()`` and ``get_FOO_width()`` methods. These are documented elsewhere_. Requires the `Python Imaging Library`_. .. _Python Imaging Library: http://www.pythonware.com/products/pil/ .. _elsewhere: ../db-api/#get-foo-height-and-get-foo-width **New in development version:** By default, ``ImageField`` instances are created as ``varchar(100)`` columns in your database. As with other fields, you can change the maximum length using the ``max_length`` argument. ``IntegerField`` ~~~~~~~~~~~~~~~~ An integer. The admin represents this as an ``<input type="text">`` (a single-line input). ``IPAddressField`` ~~~~~~~~~~~~~~~~~~ An IP address, in string format (e.g. "192.0.2.30"). The admin represents this as an ``<input type="text">`` (a single-line input). ``NullBooleanField`` ~~~~~~~~~~~~~~~~~~~~ Like a ``BooleanField``, but allows ``NULL`` as one of the options. Use this instead of a ``BooleanField`` with ``null=True``. The admin represents this as a ``<select>`` box with "Unknown", "Yes" and "No" choices. ``PhoneNumberField`` ~~~~~~~~~~~~~~~~~~~~ A ``CharField`` that checks that the value is a valid U.S.A.-style phone number (in the format ``XXX-XXX-XXXX``). ``PositiveIntegerField`` ~~~~~~~~~~~~~~~~~~~~~~~~ Like an ``IntegerField``, but must be positive. ``PositiveSmallIntegerField`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Like a ``PositiveIntegerField``, but only allows values under a certain (database-dependent) point. ``SlugField`` ~~~~~~~~~~~~~ "Slug" is a newspaper term. A slug is a short label for something, containing only letters, numbers, underscores or hyphens. They're generally used in URLs. Like a CharField, you can specify ``max_length``. If ``max_length`` is not specified, Django will use a default length of 50. Implies ``db_index=True``. Accepts an extra option, ``prepopulate_from``, which is a list of fields from which to auto-populate the slug, via JavaScript, in the object's admin form:: models.SlugField(prepopulate_from=("pre_name", "name")) ``prepopulate_from`` doesn't accept DateTimeFields, ForeignKeys nor ManyToManyFields. The admin represents ``SlugField`` as an ``<input type="text">`` (a single-line input). ``SmallIntegerField`` ~~~~~~~~~~~~~~~~~~~~~ Like an ``IntegerField``, but only allows values under a certain (database-dependent) point. ``TextField`` ~~~~~~~~~~~~~ A large text field. The admin represents this as a ``<textarea>`` (a multi-line input). ``TimeField`` ~~~~~~~~~~~~~ A time. Accepts the same auto-population options as ``DateField`` and ``DateTimeField``. The admin represents this as an ``<input type="text">`` with some JavaScript shortcuts. ``URLField`` ~~~~~~~~~~~~ A field for a URL. If the ``verify_exists`` option is ``True`` (default), the URL given will be checked for existence (i.e., the URL actually loads and doesn't give a 404 response). The admin represents this as an ``<input type="text">`` (a single-line input). ``URLField`` takes an optional argument, ``max_length``, the maximum length (in characters) of the field. The maximum length is enforced at the database level and in Django's validation. If you don't specify ``max_length``, a default of 200 is used. ``USStateField`` ~~~~~~~~~~~~~~~~ A two-letter U.S. state abbreviation. The admin represents this as an ``<input type="text">`` (a single-line input). ``XMLField`` ~~~~~~~~~~~~ A ``TextField`` that checks that the value is valid XML that matches a given schema. Takes one required argument, ``schema_path``, which is the filesystem path to a RelaxNG_ schema against which to validate the field. .. _RelaxNG: http://www.relaxng.org/ Field options ------------- The following arguments are available to all field types. All are optional. ``null`` ~~~~~~~~ If ``True``, Django will store empty values as ``NULL`` in the database. Default is ``False``. Note that empty string values will always get stored as empty strings, not as ``NULL``. Only use ``null=True`` for non-string fields such as integers, booleans and dates. For both types of fields, you will also need to set ``blank=True`` if you wish to permit empty values in forms, as the ``null`` parameter only affects database storage (see blank_, below). Avoid using ``null`` on string-based fields such as ``CharField`` and ``TextField`` unless you have an excellent reason. If a string-based field has ``null=True``, that means it has two possible values for "no data": ``NULL``, and the empty string. In most cases, it's redundant to have two possible values for "no data;" Django convention is to use the empty string, not ``NULL``. .. note:: When using the Oracle database backend, the ``null=True`` option will be coerced for string-based fields that can blank, and the value ``NULL`` will be stored to denote the empty string. ``blank`` ~~~~~~~~~ If ``True``, the field is allowed to be blank. Default is ``False``. Note that this is different than ``null``. ``null`` is purely database-related, whereas ``blank`` is validation-related. If a field has ``blank=True``, validation on Django's admin site will allow entry of an empty value. If a field has ``blank=False``, the field will be required. ``choices`` ~~~~~~~~~~~ An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field. If this is given, Django's admin will use a select box instead of the standard text field and will limit choices to the choices given. A choices list looks like this:: YEAR_IN_SCHOOL_CHOICES = ( ('FR', 'Freshman'), ('SO', 'Sophomore'), ('JR', 'Junior'), ('SR', 'Senior'), ('GR', 'Graduate'), ) The first element in each tuple is the actual value to be stored. The second element is the human-readable name for the option. The choices list can be defined either as part of your model class:: class Foo(models.Model): GENDER_CHOICES = ( ('M', 'Male'), ('F', 'Female'), ) gender = models.CharField(max_length=1, choices=GENDER_CHOICES) or outside your model class altogether:: GENDER_CHOICES = ( ('M', 'Male'), ('F', 'Female'), ) class Foo(models.Model): gender = models.CharField(max_length=1, choices=GENDER_CHOICES) For each model field that has ``choices`` set, Django will add a method to retrieve the human-readable name for the field's current value. See `get_FOO_display`_ in the database API documentation. .. _get_FOO_display: ../db-api/#get-foo-display Finally, note that choices can be any iterable object -- not necessarily a list or tuple. This lets you construct choices dynamically. But if you find yourself hacking ``choices`` to be dynamic, you're probably better off using a proper database table with a ``ForeignKey``. ``choices`` is meant for static data that doesn't change much, if ever. ``core`` ~~~~~~~~ For objects that are edited inline to a related object. In the Django admin, if all "core" fields in an inline-edited object are cleared, the object will be deleted. It is an error to have an inline-editable relation without at least one ``core=True`` field. Please note that each field marked "core" is treated as a required field by the Django admin site. Essentially, this means you should put ``core=True`` on all required fields in your related object that is being edited inline. ``db_column`` ~~~~~~~~~~~~~ The name of the database column to use for this field. If this isn't given, Django will use the field's name. If your database column name is an SQL reserved word, or contains characters that aren't allowed in Python variable names -- notably, the hyphen -- that's OK. Django quotes column and table names behind the scenes. ``db_index`` ~~~~~~~~~~~~ If ``True``, ``django-admin.py sqlindexes`` will output a ``CREATE INDEX`` statement for this field. ``db_tablespace`` ~~~~~~~~~~~~~~~~~ **New in Django development version** The name of the database tablespace to use for this field's index, if this field is indexed. The default is the project's ``DEFAULT_INDEX_TABLESPACE`` setting, if set, or the ``db_tablespace`` of the model, if any. If the backend doesn't support tablespaces, this option is ignored. ``default`` ~~~~~~~~~~~ The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created. ``editable`` ~~~~~~~~~~~~ If ``False``, the field will not be editable in the admin or via form processing using the object's ``AddManipulator`` or ``ChangeManipulator`` classes. Default is ``True``. ``help_text`` ~~~~~~~~~~~~~ Extra "help" text to be displayed under the field on the object's admin form. It's useful for documentation even if your object doesn't have an admin form. Note that this value is *not* HTML-escaped when it's displayed in the admin interface. This lets you include HTML in ``help_text`` if you so desire. For example:: help_text="Please use the following format: <em>YYYY-MM-DD</em>." ``primary_key`` ~~~~~~~~~~~~~~~ If ``True``, this field is the primary key for the model. If you don't specify ``primary_key=True`` for any fields in your model, Django will automatically add this field:: id = models.AutoField('ID', primary_key=True) Thus, you don't need to set ``primary_key=True`` on any of your fields unless you want to override the default primary-key behavior. ``primary_key=True`` implies ``null=False`` and ``unique=True``. Only one primary key is allowed on an object. ``radio_admin`` ~~~~~~~~~~~~~~~ By default, Django's admin uses a select-box interface (<select>) for fields that are ``ForeignKey`` or have ``choices`` set. If ``radio_admin`` is set to ``True``, Django will use a radio-button interface instead. Don't use this for a field unless it's a ``ForeignKey`` or has ``choices`` set. ``unique`` ~~~~~~~~~~ If ``True``, this field must be unique throughout the table. This is enforced at the database level and at the Django admin-form level. If you try to save a model with a duplicate value in a ``unique`` field, a ``django.db.IntegrityError`` will be raised by the model's ``save()`` method. ``unique_for_date`` ~~~~~~~~~~~~~~~~~~~ Set this to the name of a ``DateField`` or ``DateTimeField`` to require that this field be unique for the value of the date field. For example, if you have a field ``title`` that has ``unique_for_date="pub_date"``, then Django wouldn't allow the entry of two records with the same ``title`` and ``pub_date``. This is enforced at the Django admin-form level but not at the database level. ``unique_for_month`` ~~~~~~~~~~~~~~~~~~~~ Like ``unique_for_date``, but requires the field to be unique with respect to the month. ``unique_for_year`` ~~~~~~~~~~~~~~~~~~~ Like ``unique_for_date`` and ``unique_for_month``. ``validator_list`` ~~~~~~~~~~~~~~~~~~ A list of extra validators to apply to the field. Each should be a callable that takes the parameters ``field_data, all_data`` and raises ``django.core.validators.ValidationError`` for errors. (See the `validator docs`_.) Django comes with quite a few validators. They're in ``django.core.validators``. .. _validator docs: ../forms/#validators Verbose field names ------------------- Each field type, except for ``ForeignKey``, ``ManyToManyField`` and ``OneToOneField``, takes an optional first positional argument -- a verbose name. If the verbose name isn't given, Django will automatically create it using the field's attribute name, converting underscores to spaces. In this example, the verbose name is ``"Person's first name"``:: first_name = models.CharField("Person's first name", max_length=30) In this example, the verbose name is ``"first name"``:: first_name = models.CharField(max_length=30) ``ForeignKey``, ``ManyToManyField`` and ``OneToOneField`` require the first argument to be a model class, so use the ``verbose_name`` keyword argument:: poll = models.ForeignKey(Poll, verbose_name="the related poll") sites = models.ManyToManyField(Site, verbose_name="list of sites") place = models.OneToOneField(Place, verbose_name="related place") Convention is not to capitalize the first letter of the ``verbose_name``. Django will automatically capitalize the first letter where it needs to. Relationships ------------- Clearly, the power of relational databases lies in relating tables to each other. Django offers ways to define the three most common types of database relationships: Many-to-one, many-to-many and one-to-one. Many-to-one relationships ~~~~~~~~~~~~~~~~~~~~~~~~~ To define a many-to-one relationship, use ``ForeignKey``. You use it just like any other ``Field`` type: by including it as a class attribute of your model. ``ForeignKey`` requires a positional argument: the class to which the model is related. For example, if a ``Car`` model has a ``Manufacturer`` -- that is, a ``Manufacturer`` makes multiple cars but each ``Car`` only has one ``Manufacturer`` -- use the following definitions:: class Manufacturer(models.Model): # ... class Car(models.Model): manufacturer = models.ForeignKey(Manufacturer) # ... To create a recursive relationship -- an object that has a many-to-one relationship with itself -- use ``models.ForeignKey('self')``. If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself:: class Car(models.Model): manufacturer = models.ForeignKey('Manufacturer') # ... class Manufacturer(models.Model): # ... Note, however, that this only refers to models in the same models.py file -- you cannot use a string to reference a model defined in another application or imported from elsewhere. **New in Django development version:** To refer to models defined in another application, you must instead explicitly specify the application label. For example, if the ``Manufacturer`` model above is defined in another application called ``production``, you'd need to use:: class Car(models.Model): manufacturer = models.ForeignKey('production.Manufacturer') Behind the scenes, Django appends ``"_id"`` to the field name to create its database column name. In the above example, the database table for the ``Car`` model will have a ``manufacturer_id`` column. (You can change this explicitly by specifying ``db_column``; see ``db_column`` below.) However, your code should never have to deal with the database column name, unless you write custom SQL. You'll always deal with the field names of your model object. It's suggested, but not required, that the name of a ``ForeignKey`` field (``manufacturer`` in the example above) be the name of the model, lowercase. You can, of course, call the field whatever you want. For example:: class Car(models.Model): company_that_makes_it = models.ForeignKey(Manufacturer) # ... See the `Many-to-one relationship model example`_ for a full example. .. _Many-to-one relationship model example: ../models/many_to_one/ ``ForeignKey`` fields take a number of extra arguments for defining how the relationship should work. All are optional: ======================= ============================================================ Argument Description ======================= ============================================================ ``edit_inline`` If ``True``, this related object is edited "inline" on the related object's page. This means that the object will not have its own admin interface. Use either ``models.TABULAR`` or ``models.STACKED``, which, respectively, designate whether the inline-editable objects are displayed as a table or as a "stack" of fieldsets. ``limit_choices_to`` A dictionary of lookup arguments and values (see the `Database API reference`_) that limit the available admin choices for this object. Use this with functions from the Python ``datetime`` module to limit choices of objects by date. For example:: limit_choices_to = {'pub_date__lte': datetime.now} only allows the choice of related objects with a ``pub_date`` before the current date/time to be chosen. Instead of a dictionary this can also be a ``Q`` object (an object with a ``get_sql()`` method) for more complex queries. Not compatible with ``edit_inline``. ``max_num_in_admin`` For inline-edited objects, this is the maximum number of related objects to display in the admin. Thus, if a pizza could only have up to 10 toppings, ``max_num_in_admin=10`` would ensure that a user never enters more than 10 toppings. Note that this doesn't ensure more than 10 related toppings ever get created. It simply controls the admin interface; it doesn't enforce things at the Python API level or database level. ``min_num_in_admin`` The minimum number of related objects displayed in the admin. Normally, at the creation stage, ``num_in_admin`` inline objects are shown, and at the edit stage ``num_extra_on_change`` blank objects are shown in addition to all pre-existing related objects. However, no fewer than ``min_num_in_admin`` related objects will ever be displayed. ``num_extra_on_change`` The number of extra blank related-object fields to show at the change stage. ``num_in_admin`` The default number of inline objects to display on the object page at the add stage. ``raw_id_admin`` Only display a field for the integer to be entered instead of a drop-down menu. This is useful when related to an object type that will have too many rows to make a select box practical. Not used with ``edit_inline``. ``related_name`` The name to use for the relation from the related object back to this one. See the `related objects documentation`_ for a full explanation and example. If using this in an `abstract base class`_, be sure to read the `extra notes`_ in that section about ``related_name``. ``to_field`` The field on the related object that the relation is to. By default, Django uses the primary key of the related object. ======================= ============================================================ .. _`Database API reference`: ../db-api/ .. _related objects documentation: ../db-api/#related-objects .. _abstract base class: `Abstract base classes`_ .. _extra notes: `Be careful with related_name`_ Many-to-many relationships ~~~~~~~~~~~~~~~~~~~~~~~~~~ To define a many-to-many relationship, use ``ManyToManyField``. You use it just like any other ``Field`` type: by including it as a class attribute of your model. ``ManyToManyField`` requires a positional argument: the class to which the model is related. For example, if a ``Pizza`` has multiple ``Topping`` objects -- that is, a ``Topping`` can be on multiple pizzas and each ``Pizza`` has multiple toppings -- here's how you'd represent that:: class Topping(models.Model): # ... class Pizza(models.Model): # ... toppings = models.ManyToManyField(Topping) As with ``ForeignKey``, a relationship to self can be defined by using the string ``'self'`` instead of the model name, and you can refer to as-yet undefined models by using a string containing the model name. However, you can only use strings to refer to models in the same models.py file -- you cannot use a string to reference a model in a different application, or to reference a model that has been imported from elsewhere. It's suggested, but not required, that the name of a ``ManyToManyField`` (``toppings`` in the example above) be a plural describing the set of related model objects. Behind the scenes, Django creates an intermediary join table to represent the many-to-many relationship. It doesn't matter which model gets the ``ManyToManyField``, but you only need it in one of the models -- not in both. Generally, ``ManyToManyField`` instances should go in the object that's going to be edited in the admin interface, if you're using Django's admin. In the above example, ``toppings`` is in ``Pizza`` (rather than ``Topping`` having a ``pizzas`` ``ManyToManyField`` ) because it's more natural to think about a ``Pizza`` having toppings than a topping being on multiple pizzas. The way it's set up above, the ``Pizza`` admin form would let users select the toppings. See the `Many-to-many relationship model example`_ for a full example. .. _Many-to-many relationship model example: ../models/many_to_many/ ``ManyToManyField`` objects take a number of extra arguments for defining how the relationship should work. All are optional: ======================= ============================================================ Argument Description ======================= ============================================================ ``related_name`` See the description under ``ForeignKey`` above. ``filter_interface`` Use a nifty unobtrusive Javascript "filter" interface instead of the usability-challenged ``<select multiple>`` in the admin form for this object. The value should be ``models.HORIZONTAL`` or ``models.VERTICAL`` (i.e. should the interface be stacked horizontally or vertically). ``limit_choices_to`` See the description under ``ForeignKey`` above. ``symmetrical`` Only used in the definition of ManyToManyFields on self. Consider the following model:: class Person(models.Model): friends = models.ManyToManyField("self") When Django processes this model, it identifies that it has a ``ManyToManyField`` on itself, and as a result, it doesn't add a ``person_set`` attribute to the ``Person`` class. Instead, the ``ManyToManyField`` is assumed to be symmetrical -- that is, if I am your friend, then you are my friend. If you do not want symmetry in ``ManyToMany`` relationships with ``self``, set ``symmetrical`` to ``False``. This will force Django to add the descriptor for the reverse relationship, allowing ``ManyToMany`` relationships to be non-symmetrical. ``db_table`` The name of the table to create for storing the many-to-many data. If this is not provided, Django will assume a default name based upon the names of the two tables being joined. ======================= ============================================================ One-to-one relationships ~~~~~~~~~~~~~~~~~~~~~~~~ To define a one-to-one relationship, use ``OneToOneField``. You use it just like any other ``Field`` type: by including it as a class attribute of your model. This is most useful on the primary key of an object when that object "extends" another object in some way. ``OneToOneField`` requires a positional argument: the class to which the model is related. For example, if you're building a database of "places", you would build pretty standard stuff such as address, phone number, etc. in the database. Then, if you wanted to build a database of restaurants on top of the places, instead of repeating yourself and replicating those fields in the ``Restaurant`` model, you could make ``Restaurant`` have a ``OneToOneField`` to ``Place`` (because a restaurant "is-a" place). As with ``ForeignKey``, a relationship to self can be defined by using the string ``"self"`` instead of the model name; references to as-yet undefined models can be made by using a string containing the model name. Finally, ``OneToOneField`` takes the following extra option: ======================= ============================================================ Argument Description ======================= ============================================================ ``parent_link`` When ``True`` and used in a model inherited from another model, indicates that this field should be used as the link from the child back to the parent. See `Model inheritance`_ for more details. **New in Django development version** ======================= ============================================================ **New in Django development version:** ``OneToOneField`` classes used to automatically become the primary key on a model. This is no longer true, although you can manually pass in the ``primary_key`` attribute if you like. Thus, it's now possible to have multiple fields of type ``OneToOneField`` on a single model. See the `One-to-one relationship model example`_ for a full example. .. _One-to-one relationship model example: ../models/one_to_one/ Custom field types ------------------ **New in Django development version** If one of the existing model fields cannot be used to fit your purposes, or if you wish to take advantage of some less common database column types, you can create your own field class. Full coverage of creating your own fields is provided in the `Custom Model Fields`_ documentation. .. _Custom Model Fields: ../custom_model_fields/ Meta options ============ Give your model metadata by using an inner ``class Meta``, like so:: class Foo(models.Model): bar = models.CharField(max_length=30) class Meta: # ... Model metadata is "anything that's not a field", such as ordering options, etc. Here's a list of all possible ``Meta`` options. No options are required. Adding ``class Meta`` to a model is completely optional. ``abstract`` ------------ **New in Django development version** When set to ``True``, denotes this model as an abstract base class. See `Abstract base classes`_ for more details. Defaults to ``False``. ``db_table`` ------------ The name of the database table to use for the model:: db_table = 'music_album' If this isn't given, Django will use ``app_label + '_' + model_class_name``. See "Table names" below for more. If your database table name is an SQL reserved word, or contains characters that aren't allowed in Python variable names -- notably, the hyphen -- that's OK. Django quotes column and table names behind the scenes. ``db_tablespace`` ----------------- **New in Django development version** The name of the database tablespace to use for the model. If the backend doesn't support tablespaces, this option is ignored. ``get_latest_by`` ----------------- The name of a ``DateField`` or ``DateTimeField`` in the model. This specifies the default field to use in your model ``Manager``'s ``latest()`` method. Example:: get_latest_by = "order_date" See the `docs for latest()`_ for more. .. _docs for latest(): ../db-api/#latest-field-name-none ``order_with_respect_to`` ------------------------- Marks this object as "orderable" with respect to the given field. This is almost always used with related objects to allow them to be ordered with respect to a parent object. For example, if an ``Answer`` relates to a ``Question`` object, and a question has more than one answer, and the order of answers matters, you'd do this:: class Answer(models.Model): question = models.ForeignKey(Question) # ... class Meta: order_with_respect_to = 'question' ``ordering`` ------------ The default ordering for the object, for use when obtaining lists of objects:: ordering = ['-order_date'] This is a tuple or list of strings. Each string is a field name with an optional "-" prefix, which indicates descending order. Fields without a leading "-" will be ordered ascending. Use the string "?" to order randomly. For example, to order by a ``pub_date`` field ascending, use this:: ordering = ['pub_date'] To order by ``pub_date`` descending, use this:: ordering = ['-pub_date'] To order by ``pub_date`` descending, then by ``author`` ascending, use this:: ordering = ['-pub_date', 'author'] See `Specifying ordering`_ for more examples. Note that, regardless of how many fields are in ``ordering``, the admin site uses only the first field. .. _Specifying ordering: ../models/ordering/ ``permissions`` --------------- Extra permissions to enter into the permissions table when creating this object. Add, delete and change permissions are automatically created for each object that has ``admin`` set. This example specifies an extra permission, ``can_deliver_pizzas``:: permissions = (("can_deliver_pizzas", "Can deliver pizzas"),) This is a list or tuple of 2-tuples in the format ``(permission_code, human_readable_permission_name)``. ``unique_together`` ------------------- Sets of field names that, taken together, must be unique:: unique_together = (("driver", "restaurant"),) This is a list of lists of fields that must be unique when considered together. It's used in the Django admin and is enforced at the database level (i.e., the appropriate ``UNIQUE`` statements are included in the ``CREATE TABLE`` statement). All the fields specified in ``unique_together`` must be part of the current model. If you are using `model inheritance`_, you cannot refer to fields from any parent classes in ``unique_together``. **New in Django development version** For convenience, unique_together can be a single list when dealing with a single set of fields:: unique_together = ("driver", "restaurant") ``verbose_name`` ---------------- A human-readable name for the object, singular:: verbose_name = "pizza" If this isn't given, Django will use a munged version of the class name: ``CamelCase`` becomes ``camel case``. ``verbose_name_plural`` ----------------------- The plural name for the object:: verbose_name_plural = "stories" If this isn't given, Django will use ``verbose_name + "s"``. Table names =========== To save you time, Django automatically derives the name of the database table from the name of your model class and the app that contains it. A model's database table name is constructed by joining the model's "app label" -- the name you used in ``manage.py startapp`` -- to the model's class name, with an underscore between them. For example, if you have an app ``bookstore`` (as created by ``manage.py startapp bookstore``), a model defined as ``class Book`` will have a database table named ``bookstore_book``. To override the database table name, use the ``db_table`` parameter in ``class Meta``. Automatic primary key fields ============================ By default, Django gives each model the following field:: id = models.AutoField(primary_key=True) This is an auto-incrementing primary key. If you'd like to specify a custom primary key, just specify ``primary_key=True`` on one of your fields. If Django sees you've explicitly set ``primary_key``, it won't add the automatic ``id`` column. Each model requires exactly one field to have ``primary_key=True``. The ``pk`` property ------------------- **New in Django development version** Regardless of whether you define a primary key field yourself, or let Django supply one for you, each model will have a property called ``pk``. It behaves like a normal attribute on the model, but is actually an alias for whichever attribute is the primary key field for the model. You can read and set this value, just as you would for any other attribute, and it will update the correct field in the model. Admin options ============= If you want your model to be visible to Django's admin site, give your model an inner ``"class Admin"``, like so:: class Person(models.Model): first_name = models.CharField(max_length=30) last_name = models.CharField(max_length=30) class Admin: # Admin options go here pass The ``Admin`` class tells Django how to display the model in the admin site. Here's a list of all possible ``Admin`` options. None of these options are required. To use an admin interface without specifying any options, use ``pass``, like so:: class Admin: pass Adding ``class Admin`` to a model is completely optional. ``date_hierarchy`` ------------------ Set ``date_hierarchy`` to the name of a ``DateField`` or ``DateTimeField`` in your model, and the change list page will include a date-based drilldown navigation by that field. Example:: date_hierarchy = 'pub_date' ``fields`` ---------- Set ``fields`` to control the layout of admin "add" and "change" pages. ``fields`` is a list of two-tuples, in which each two-tuple represents a ``<fieldset>`` on the admin form page. (A ``<fieldset>`` is a "section" of the form.) The two-tuples are in the format ``(name, field_options)``, where ``name`` is a string representing the title of the fieldset and ``field_options`` is a dictionary of information about the fieldset, including a list of fields to be displayed in it. A full example, taken from the ``django.contrib.flatpages.FlatPage`` model:: class Admin: fields = ( (None, { 'fields': ('url', 'title', 'content', 'sites') }), ('Advanced options', { 'classes': 'collapse', 'fields' : ('enable_comments', 'registration_required', 'template_name') }), ) This results in an admin page that looks like: .. image:: http://media.djangoproject.com/img/doc/flatfiles_admin.png If ``fields`` isn't given, Django will default to displaying each field that isn't an ``AutoField`` and has ``editable=True``, in a single fieldset, in the same order as the fields are defined in the model. The ``field_options`` dictionary can have the following keys: ``fields`` ~~~~~~~~~~ A tuple of field names to display in this fieldset. This key is required. Example:: { 'fields': ('first_name', 'last_name', 'address', 'city', 'state'), } To display multiple fields on the same line, wrap those fields in their own tuple. In this example, the ``first_name`` and ``last_name`` fields will display on the same line:: { 'fields': (('first_name', 'last_name'), 'address', 'city', 'state'), } ``classes`` ~~~~~~~~~~~ A string containing extra CSS classes to apply to the fieldset. Example:: { 'classes': 'wide', } Apply multiple classes by separating them with spaces. Example:: { 'classes': 'wide extrapretty', } Two useful classes defined by the default admin-site stylesheet are ``collapse`` and ``wide``. Fieldsets with the ``collapse`` style will be initially collapsed in the admin and replaced with a small "click to expand" link. Fieldsets with the ``wide`` style will be given extra horizontal space. ``description`` ~~~~~~~~~~~~~~~ A string of optional extra text to be displayed at the top of each fieldset, under the heading of the fieldset. It's used verbatim, so you can use any HTML and you must escape any special HTML characters (such as ampersands) yourself. ``js`` ------ A list of strings representing URLs of JavaScript files to link into the admin screen via ``<script src="">`` tags. This can be used to tweak a given type of admin page in JavaScript or to provide "quick links" to fill in default values for certain fields. If you use relative URLs -- URLs that don't start with ``http://`` or ``/`` -- then the admin site will automatically prefix these links with ``settings.ADMIN_MEDIA_PREFIX``. ``list_display`` ---------------- Set ``list_display`` to control which fields are displayed on the change list page of the admin. Example:: list_display = ('first_name', 'last_name') If you don't set ``list_display``, the admin site will display a single column that displays the ``__str__()`` representation of each object. A few special cases to note about ``list_display``: * If the field is a ``ForeignKey``, Django will display the ``__unicode__()`` of the related object. * ``ManyToManyField`` fields aren't supported, because that would entail executing a separate SQL statement for each row in the table. If you want to do this nonetheless, give your model a custom method, and add that method's name to ``list_display``. (See below for more on custom methods in ``list_display``.) * If the field is a ``BooleanField`` or ``NullBooleanField``, Django will display a pretty "on" or "off" icon instead of ``True`` or ``False``. * If the string given is a method of the model, Django will call it and display the output. This method should have a ``short_description`` function attribute, for use as the header for the field. Here's a full example model:: class Person(models.Model): name = models.CharField(max_length=50) birthday = models.DateField() class Admin: list_display = ('name', 'decade_born_in') def decade_born_in(self): return self.birthday.strftime('%Y')[:3] + "0's" decade_born_in.short_description = 'Birth decade' * If the string given is a method of the model, Django will HTML-escape the output by default. If you'd rather not escape the output of the method, give the method an ``allow_tags`` attribute whose value is ``True``. Here's a full example model:: class Person(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) color_code = models.CharField(max_length=6) class Admin: list_display = ('first_name', 'last_name', 'colored_name') def colored_name(self): return '<span style="color: #%s;">%s %s</span>' % (self.color_code, self.first_name, self.last_name) colored_name.allow_tags = True * If the string given is a method of the model that returns True or False Django will display a pretty "on" or "off" icon if you give the method a ``boolean`` attribute whose value is ``True``. Here's a full example model:: class Person(models.Model): first_name = models.CharField(max_length=50) birthday = models.DateField() class Admin: list_display = ('name', 'born_in_fifties') def born_in_fifties(self): return self.birthday.strftime('%Y')[:3] == 5 born_in_fifties.boolean = True * The ``__str__()`` and ``__unicode__()`` methods are just as valid in ``list_display`` as any other model method, so it's perfectly OK to do this:: list_display = ('__unicode__', 'some_other_field') * Usually, elements of ``list_display`` that aren't actual database fields can't be used in sorting (because Django does all the sorting at the database level). However, if an element of ``list_display`` represents a certain database field, you can indicate this fact by setting the ``admin_order_field`` attribute of the item. For example:: class Person(models.Model): first_name = models.CharField(max_length=50) color_code = models.CharField(max_length=6) class Admin: list_display = ('first_name', 'colored_first_name') def colored_first_name(self): return '<span style="color: #%s;">%s</span>' % (self.color_code, self.first_name) colored_first_name.allow_tags = True colored_first_name.admin_order_field = 'first_name' The above will tell Django to order by the ``first_name`` field when trying to sort by ``colored_first_name`` in the admin. ``list_display_links`` ---------------------- Set ``list_display_links`` to control which fields in ``list_display`` should be linked to the "change" page for an object. By default, the change list page will link the first column -- the first field specified in ``list_display`` -- to the change page for each item. But ``list_display_links`` lets you change which columns are linked. Set ``list_display_links`` to a list or tuple of field names (in the same format as ``list_display``) to link. ``list_display_links`` can specify one or many field names. As long as the field names appear in ``list_display``, Django doesn't care how many (or how few) fields are linked. The only requirement is: If you want to use ``list_display_links``, you must define ``list_display``. In this example, the ``first_name`` and ``last_name`` fields will be linked on the change list page:: class Admin: list_display = ('first_name', 'last_name', 'birthday') list_display_links = ('first_name', 'last_name') Finally, note that in order to use ``list_display_links``, you must define ``list_display``, too. ``list_filter`` --------------- Set ``list_filter`` to activate filters in the right sidebar of the change list page of the admin. This should be a list of field names, and each specified field should be either a ``BooleanField``, ``CharField``, ``DateField``, ``DateTimeField``, ``IntegerField`` or ``ForeignKey``. This example, taken from the ``django.contrib.auth.models.User`` model, shows how both ``list_display`` and ``list_filter`` work:: class Admin: list_display = ('username', 'email', 'first_name', 'last_name', 'is_staff') list_filter = ('is_staff', 'is_superuser') The above code results in an admin change list page that looks like this: .. image:: http://media.djangoproject.com/img/doc/users_changelist.png (This example also has ``search_fields`` defined. See below.) ``list_per_page`` ----------------- Set ``list_per_page`` to control how many items appear on each paginated admin change list page. By default, this is set to ``100``. ``list_select_related`` ----------------------- Set ``list_select_related`` to tell Django to use ``select_related()`` in retrieving the list of objects on the admin change list page. This can save you a bunch of database queries. The value should be either ``True`` or ``False``. Default is ``False``. Note that Django will use ``select_related()``, regardless of this setting, if one of the ``list_display`` fields is a ``ForeignKey``. For more on ``select_related()``, see `the select_related() docs`_. .. _the select_related() docs: ../db-api/#select-related ``ordering`` ------------ Set ``ordering`` to specify how objects on the admin change list page should be ordered. This should be a list or tuple in the same format as a model's ``ordering`` parameter. If this isn't provided, the Django admin will use the model's default ordering. ``save_as`` ----------- Set ``save_as`` to enable a "save as" feature on admin change forms. Normally, objects have three save options: "Save", "Save and continue editing" and "Save and add another". If ``save_as`` is ``True``, "Save and add another" will be replaced by a "Save as" button. "Save as" means the object will be saved as a new object (with a new ID), rather than the old object. By default, ``save_as`` is set to ``False``. ``save_on_top`` --------------- Set ``save_on_top`` to add save buttons across the top of your admin change forms. Normally, the save buttons appear only at the bottom of the forms. If you set ``save_on_top``, the buttons will appear both on the top and the bottom. By default, ``save_on_top`` is set to ``False``. ``search_fields`` ----------------- Set ``search_fields`` to enable a search box on the admin change list page. This should be set to a list of field names that will be searched whenever somebody submits a search query in that text box. These fields should be some kind of text field, such as ``CharField`` or ``TextField``. You can also perform a related lookup on a ``ForeignKey`` with the lookup API "follow" notation:: search_fields = ['foreign_key__related_fieldname'] When somebody does a search in the admin search box, Django splits the search query into words and returns all objects that contain each of the words, case insensitive, where each word must be in at least one of ``search_fields``. For example, if ``search_fields`` is set to ``['first_name', 'last_name']`` and a user searches for ``john lennon``, Django will do the equivalent of this SQL ``WHERE`` clause:: WHERE (first_name ILIKE '%john%' OR last_name ILIKE '%john%') AND (first_name ILIKE '%lennon%' OR last_name ILIKE '%lennon%') For faster and/or more restrictive searches, prefix the field name with an operator: ``^`` Matches the beginning of the field. For example, if ``search_fields`` is set to ``['^first_name', '^last_name']`` and a user searches for ``john lennon``, Django will do the equivalent of this SQL ``WHERE`` clause:: WHERE (first_name ILIKE 'john%' OR last_name ILIKE 'john%') AND (first_name ILIKE 'lennon%' OR last_name ILIKE 'lennon%') This query is more efficient than the normal ``'%john%'`` query, because the database only needs to check the beginning of a column's data, rather than seeking through the entire column's data. Plus, if the column has an index on it, some databases may be able to use the index for this query, even though it's a ``LIKE`` query. ``=`` Matches exactly, case-insensitive. For example, if ``search_fields`` is set to ``['=first_name', '=last_name']`` and a user searches for ``john lennon``, Django will do the equivalent of this SQL ``WHERE`` clause:: WHERE (first_name ILIKE 'john' OR last_name ILIKE 'john') AND (first_name ILIKE 'lennon' OR last_name ILIKE 'lennon') Note that the query input is split by spaces, so, following this example, it's currently not possible to search for all records in which ``first_name`` is exactly ``'john winston'`` (containing a space). ``@`` Performs a full-text match. This is like the default search method but uses an index. Currently this is only available for MySQL. Managers ======== A ``Manager`` is the interface through which database query operations are provided to Django models. At least one ``Manager`` exists for every model in a Django application. The way ``Manager`` classes work is documented in the `Retrieving objects`_ section of the database API docs, but this section specifically touches on model options that customize ``Manager`` behavior. .. _Retrieving objects: ../db-api/#retrieving-objects Manager names ------------- By default, Django adds a ``Manager`` with the name ``objects`` to every Django model class. However, if you want to use ``objects`` as a field name, or if you want to use a name other than ``objects`` for the ``Manager``, you can rename it on a per-model basis. To rename the ``Manager`` for a given class, define a class attribute of type ``models.Manager()`` on that model. For example:: from django.db import models class Person(models.Model): #... people = models.Manager() Using this example model, ``Person.objects`` will generate an ``AttributeError`` exception, but ``Person.people.all()`` will provide a list of all ``Person`` objects. Custom Managers --------------- You can use a custom ``Manager`` in a particular model by extending the base ``Manager`` class and instantiating your custom ``Manager`` in your model. There are two reasons you might want to customize a ``Manager``: to add extra ``Manager`` methods, and/or to modify the initial ``QuerySet`` the ``Manager`` returns. Adding extra Manager methods ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Adding extra ``Manager`` methods is the preferred way to add "table-level" functionality to your models. (For "row-level" functionality -- i.e., functions that act on a single instance of a model object -- use `Model methods`_, not custom ``Manager`` methods.) A custom ``Manager`` method can return anything you want. It doesn't have to return a ``QuerySet``. For example, this custom ``Manager`` offers a method ``with_counts()``, which returns a list of all ``OpinionPoll`` objects, each with an extra ``num_responses`` attribute that is the result of an aggregate query:: class PollManager(models.Manager): def with_counts(self): from django.db import connection cursor = connection.cursor() cursor.execute(""" SELECT p.id, p.question, p.poll_date, COUNT(*) FROM polls_opinionpoll p, polls_response r WHERE p.id = r.poll_id GROUP BY 1, 2, 3 ORDER BY 3 DESC""") result_list = [] for row in cursor.fetchall(): p = self.model(id=row[0], question=row[1], poll_date=row[2]) p.num_responses = row[3] result_list.append(p) return result_list class OpinionPoll(models.Model): question = models.CharField(max_length=200) poll_date = models.DateField() objects = PollManager() class Response(models.Model): poll = models.ForeignKey(Poll) person_name = models.CharField(max_length=50) response = models.TextField() With this example, you'd use ``OpinionPoll.objects.with_counts()`` to return that list of ``OpinionPoll`` objects with ``num_responses`` attributes. Another thing to note about this example is that ``Manager`` methods can access ``self.model`` to get the model class to which they're attached. Modifying initial Manager QuerySets ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A ``Manager``'s base ``QuerySet`` returns all objects in the system. For example, using this model:: class Book(models.Model): title = models.CharField(max_length=100) author = models.CharField(max_length=50) ...the statement ``Book.objects.all()`` will return all books in the database. You can override a ``Manager``\'s base ``QuerySet`` by overriding the ``Manager.get_query_set()`` method. ``get_query_set()`` should return a ``QuerySet`` with the properties you require. For example, the following model has *two* ``Manager``\s -- one that returns all objects, and one that returns only the books by Roald Dahl:: # First, define the Manager subclass. class DahlBookManager(models.Manager): def get_query_set(self): return super(DahlBookManager, self).get_query_set().filter(author='Roald Dahl') # Then hook it into the Book model explicitly. class Book(models.Model): title = models.CharField(max_length=100) author = models.CharField(max_length=50) objects = models.Manager() # The default manager. dahl_objects = DahlBookManager() # The Dahl-specific manager. With this sample model, ``Book.objects.all()`` will return all books in the database, but ``Book.dahl_objects.all()`` will only return the ones written by Roald Dahl. Of course, because ``get_query_set()`` returns a ``QuerySet`` object, you can use ``filter()``, ``exclude()`` and all the other ``QuerySet`` methods on it. So these statements are all legal:: Book.dahl_objects.all() Book.dahl_objects.filter(title='Matilda') Book.dahl_objects.count() This example also pointed out another interesting technique: using multiple managers on the same model. You can attach as many ``Manager()`` instances to a model as you'd like. This is an easy way to define common "filters" for your models. For example:: class MaleManager(models.Manager): def get_query_set(self): return super(MaleManager, self).get_query_set().filter(sex='M') class FemaleManager(models.Manager): def get_query_set(self): return super(FemaleManager, self).get_query_set().filter(sex='F') class Person(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) sex = models.CharField(max_length=1, choices=(('M', 'Male'), ('F', 'Female'))) people = models.Manager() men = MaleManager() women = FemaleManager() This example allows you to request ``Person.men.all()``, ``Person.women.all()``, and ``Person.people.all()``, yielding predictable results. If you use custom ``Manager`` objects, take note that the first ``Manager`` Django encounters (in the order in which they're defined in the model) has a special status. Django interprets this first ``Manager`` defined in a class as the "default" ``Manager``, and several parts of Django (though not the admin application) will use that ``Manager`` exclusively for that model. As a result, it's often a good idea to be careful in your choice of default manager, in order to avoid a situation where overriding of ``get_query_set()`` results in an inability to retrieve objects you'd like to work with. Model methods ============= Define custom methods on a model to add custom "row-level" functionality to your objects. Whereas ``Manager`` methods are intended to do "table-wide" things, model methods should act on a particular model instance. This is a valuable technique for keeping business logic in one place -- the model. For example, this model has a few custom methods:: class Person(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) birth_date = models.DateField() address = models.CharField(max_length=100) city = models.CharField(max_length=50) state = models.USStateField() # Yes, this is America-centric... def baby_boomer_status(self): "Returns the person's baby-boomer status." import datetime if datetime.date(1945, 8, 1) <= self.birth_date <= datetime.date(1964, 12, 31): return "Baby boomer" if self.birth_date < datetime.date(1945, 8, 1): return "Pre-boomer" return "Post-boomer" def is_midwestern(self): "Returns True if this person is from the Midwest." return self.state in ('IL', 'WI', 'MI', 'IN', 'OH', 'IA', 'MO') def _get_full_name(self): "Returns the person's full name." return '%s %s' % (self.first_name, self.last_name) full_name = property(_get_full_name) The last method in this example is a *property*. `Read more about properties`_. .. _Read more about properties: http://www.python.org/download/releases/2.2/descrintro/#property A few object methods have special meaning: ``__str__`` ----------- ``__str__()`` is a Python "magic method" that defines what should be returned if you call ``str()`` on the object. Django uses ``str(obj)`` (or the related function, ``unicode(obj)`` -- see below) in a number of places, most notably as the value displayed to render an object in the Django admin site and as the value inserted into a template when it displays an object. Thus, you should always return a nice, human-readable string for the object's ``__str__``. Although this isn't required, it's strongly encouraged (see the description of ``__unicode__``, below, before putting ``__str__`` methods everywhere). For example:: class Person(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) def __str__(self): # Note use of django.utils.encoding.smart_str() here because # first_name and last_name will be unicode strings. return smart_str('%s %s' % (self.first_name, self.last_name)) ``__unicode__`` --------------- The ``__unicode__()`` method is called whenever you call ``unicode()`` on an object. Since Django's database backends will return Unicode strings in your model's attributes, you would normally want to write a ``__unicode__()`` method for your model. The example in the previous section could be written more simply as:: class Person(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) def __unicode__(self): return u'%s %s' % (self.first_name, self.last_name) If you define a ``__unicode__()`` method on your model and not a ``__str__()`` method, Django will automatically provide you with a ``__str__()`` that calls ``__unicode__()`` and then converts the result correctly to a UTF-8 encoded string object. This is recommended development practice: define only ``__unicode__()`` and let Django take care of the conversion to string objects when required. ``get_absolute_url`` -------------------- Define a ``get_absolute_url()`` method to tell Django how to calculate the URL for an object. For example:: def get_absolute_url(self): return "/people/%i/" % self.id Django uses this in its admin interface. If an object defines ``get_absolute_url()``, the object-editing page will have a "View on site" link that will jump you directly to the object's public view, according to ``get_absolute_url()``. Also, a couple of other bits of Django, such as the `syndication feed framework`_, use ``get_absolute_url()`` as a convenience to reward people who've defined the method. .. _syndication feed framework: ../syndication_feeds/ It's good practice to use ``get_absolute_url()`` in templates, instead of hard-coding your objects' URLs. For example, this template code is bad:: <a href="/people/{{ object.id }}/">{{ object.name }}</a> But this template code is good:: <a href="{{ object.get_absolute_url }}">{{ object.name }}</a> .. note:: The string you return from ``get_absolute_url()`` must contain only ASCII characters (required by the URI spec, `RFC 2396`_) that have been URL-encoded, if necessary. Code and templates using ``get_absolute_url()`` should be able to use the result directly without needing to do any further processing. You may wish to use the ``django.utils.encoding.iri_to_uri()`` function to help with this if you are using unicode strings a lot. .. _RFC 2396: http://www.ietf.org/rfc/rfc2396.txt The ``permalink`` decorator ~~~~~~~~~~~~~~~~~~~~~~~~~~~ The problem with the way we wrote ``get_absolute_url()`` above is that it slightly violates the DRY principle: the URL for this object is defined both in the URLConf file and in the model. You can further decouple your models from the URLconf using the ``permalink`` decorator. This decorator is passed the view function, a list of positional parameters and (optionally) a dictionary of named parameters. Django then works out the correct full URL path using the URLconf, substituting the parameters you have given into the URL. For example, if your URLconf contained a line such as:: (r'^people/(\d+)/$', 'people.views.details'), ...your model could have a ``get_absolute_url`` method that looked like this:: from django.db.models import permalink def get_absolute_url(self): return ('people.views.details', [str(self.id)]) get_absolute_url = permalink(get_absolute_url) Similarly, if you had a URLconf entry that looked like:: (r'/archive/(?P<year>\d{4})/(?P<month>\d{1,2})/(?P<day>\d{1,2})/$', archive_view) ...you could reference this using ``permalink()`` as follows:: def get_absolute_url(self): return ('archive_view', (), { 'year': self.created.year, 'month': self.created.month, 'day': self.created.day}) get_absolute_url = permalink(get_absolute_url) Notice that we specify an empty sequence for the second parameter in this case, because we only want to pass keyword parameters, not positional ones. In this way, you're tying the model's absolute URL to the view that is used to display it, without repeating the URL information anywhere. You can still use the ``get_absolute_url`` method in templates, as before. In some cases, such as the use of generic views or the re-use of custom views for multiple models, specifying the view function may confuse the reverse URL matcher (because multiple patterns point to the same view). For that problem, Django has **named URL patterns**. Using a named URL pattern, it's possible to give a name to a pattern, and then reference the name rather than the view function. A named URL pattern is defined by replacing the pattern tuple by a call to the ``url`` function):: from django.conf.urls.defaults import * url(r'^people/(\d+)/$', 'django.views.generic.list_detail.object_detail', name='people_view'), ...and then using that name to perform the reverse URL resolution instead of the view name:: from django.db.models import permalink def get_absolute_url(self): return ('people_view', [str(self.id)]) get_absolute_url = permalink(get_absolute_url) More details on named URL patterns are in the `URL dispatch documentation`_. .. _URL dispatch documentation: ../url_dispatch/#naming-url-patterns Executing custom SQL -------------------- Feel free to write custom SQL statements in custom model methods and module-level methods. The object ``django.db.connection`` represents the current database connection. To use it, call ``connection.cursor()`` to get a cursor object. Then, call ``cursor.execute(sql, [params])`` to execute the SQL and ``cursor.fetchone()`` or ``cursor.fetchall()`` to return the resulting rows. Example:: def my_custom_sql(self): from django.db import connection cursor = connection.cursor() cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz]) row = cursor.fetchone() return row ``connection`` and ``cursor`` mostly implement the standard `Python DB-API`_ (except when it comes to `transaction handling`_). If you're not familiar with the Python DB-API, note that the SQL statement in ``cursor.execute()`` uses placeholders, ``"%s"``, rather than adding parameters directly within the SQL. If you use this technique, the underlying database library will automatically add quotes and escaping to your parameter(s) as necessary. (Also note that Django expects the ``"%s"`` placeholder, *not* the ``"?"`` placeholder, which is used by the SQLite Python bindings. This is for the sake of consistency and sanity.) A final note: If all you want to do is a custom ``WHERE`` clause, you can just use the ``where``, ``tables`` and ``params`` arguments to the standard lookup API. See `Other lookup options`_. .. _Python DB-API: http://www.python.org/peps/pep-0249.html .. _Other lookup options: ../db-api/#extra-select-none-where-none-params-none-tables-none .. _transaction handling: ../transactions/ Overriding default model methods -------------------------------- As explained in the `database API docs`_, each model gets a few methods automatically -- most notably, ``save()`` and ``delete()``. You can override these methods to alter behavior. A classic use-case for overriding the built-in methods is if you want something to happen whenever you save an object. For example:: class Blog(models.Model): name = models.CharField(max_length=100) tagline = models.TextField() def save(self): do_something() super(Blog, self).save() # Call the "real" save() method. do_something_else() You can also prevent saving:: class Blog(models.Model): name = models.CharField(max_length=100) tagline = models.TextField() def save(self): if self.name == "Yoko Ono's blog": return # Yoko shall never have her own blog! else: super(Blog, self).save() # Call the "real" save() method. .. _database API docs: ../db-api/ Model inheritance ================= **New in Django development version** Model inheritance in Django works almost identically to the way normal class inheritance works in Python. The only decision you have to make is whether you want the parent models to be models in their own right (with their own database tables), or if the parents are just holders of common information that will only be visible through the child models. Often, you will just want to use the parent class to hold information that you don't want to have to type out for each child model. This class isn't going to ever be used in isolation, so `abstract base classes`_ are what you're after. However, if you're subclassing an existing model (perhaps something from another application entirely), or want each model to have its own database table, `multi-table inheritance`_ is the way to go. Abstract base classes --------------------- Abstract base classes are useful when you want to put some common information into a number of other models. You write your base class and put ``abstract=True`` in the ``Meta`` class. This model will then not be used to create any database table. Instead, when it is used as a base class for other models, its fields will be added to those of the child class. It is an error to have fields in the abstract base class with the same name as those in the child (and Django will raise an exception). An example:: class CommonInfo(models.Model): name = models.CharField(max_length=100) age = models.PositiveIntegerField() class Meta: abstract = True class Student(CommonInfo): home_group = models.CharField(max_length=5) The ``Student`` model will have three fields: ``name``, ``age`` and ``home_group``. The ``CommonInfo`` model cannot be used as a normal Django model, since it is an abstract base class. It does not generate a database table or have a manager or anything like that. For many uses, this type of model inheritance will be exactly what you want. It provides a way to factor out common information at the Python level, whilst still only creating one database table per child model at the database level. ``Meta`` inheritance ~~~~~~~~~~~~~~~~~~~~ When an abstract base class is created, Django makes any ``Meta`` inner class you declared on the base class available as an attribute. If a child class does not declare its own ``Meta`` class, it will inherit the parent's ``Meta``. If the child wants to extend the parent's ``Meta`` class, it can subclass it. For example:: class CommonInfo(models.Model): ... class Meta: abstract = True ordering = ['name'] class Student(CommonInfo): ... class Meta(CommonInfo.Meta): db_table = 'student_info' Django does make one adjustment to the ``Meta`` class of an abstract base class: before installing the ``Meta`` attribute, it sets ``abstract=False``. This means that children of abstract base classes don't automatically become abstract classes themselves. Of course, you can make an abstract base class that inherits from another abstract base class. You just need to remember to explicitly set ``abstract=True`` each time. Some attributes won't make sense to include in the ``Meta`` class of an abstract base class. For example, including ``db_table`` would mean that all the child classes (the ones that don't specify their own ``Meta``) would use the same database table, which is almost certainly not what you want. Be careful with ``related_name`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If you are using the ``related_name`` attribute on a ``ForeignKey`` or ``ManyToManyField``, you must always specify a *unique* reverse name for the field. This would normally cause a problem in abstract base classes, since the fields on this class are included into each of the child classes, with exactly the same values for the attributes (including ``related_name``) each time. To work around this problem, when you are using ``related_name`` in an abstract base class (only), part of the name should be the string ``'%(class)s'``. This is replaced by the lower-cased name of the child class that the field is used in. Since each class has a different name, each related name will end up being different. For example:: class Base(models.Model): m2m = models.ManyToMany(OtherModel, related_name="%(class)s_related") class Meta: abstract = True class ChildA(Base): pass class ChildB(Base): pass The reverse name of the ``ChildA.m2m`` field will be ``childa_related``, whilst the reverse name of the ``ChildB.m2m`` field will be ``childb_related``. It is up to you how you use the ``'%(class)s'`` portion to construct your related name, but if you forget to use it, Django will raise errors when you validate your models (or run ``syncdb``). If you don't specify a ``related_name`` attribute for a field in an abstract base class, the default reverse name will be the name of the child class followed by ``'_set'``, just as it normally would be if you'd declared the field directly on the child class. For example, in the above code, if the ``related_name`` attribute was omitted, the reverse name for the ``m2m`` field would be ``childa_set`` in the ``ChildA`` case and ``childb_set`` for the ``ChildB`` field. Multi-table inheritance ----------------------- The second type of model inheritance supported by Django is when each model in the hierarchy is a model all by itself. Each model corresponds to its own database table and can be queried and created individually. The inheritance relationship introduces links between the child model and each of its parents (via an automatically created ``OneToOneField``). For example:: class Place(models.Model): name = models.CharField(max_length=50) address = models.CharField(max_length=80) class Restaurant(Place): serves_hot_dogs = models.BooleanField() serves_pizza = models.BooleanField() All of the fields of ``Place`` will also be available in ``Restaurant``, although the data will reside in a different database table. So these are both possible:: >>> Place.objects.filter(name="Bob's Cafe") >>> Restaurant.objects.filter(name="Bob's Cafe") If you have a ``Place`` that is also a ``Restaurant``, you can get from the ``Place`` object to the ``Restaurant`` object by using the lower-case version of the model name:: >>> p = Place.objects.filter(name="Bob's Cafe") # If Bob's Cafe is a Restaurant object, this will give the child class: >>> p.restaurant <Restaurant: ...> However, if ``p`` in the above example was *not* a ``Restaurant`` (it had been created directly as a ``Place`` object or was the parent of some other class), referring to ``p.restaurant`` would give an error. ``Meta`` and multi-table inheritance ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the multi-table inheritance situation, it doesn't make sense for a child class to inherit from its parent's ``Meta`` class. All the ``Meta`` options have already been applied to the parent class and applying them again would normally only lead to contradictory behaviour (this is in contrast with the abstract base class case, where the base class doesn't exist in its own right). So a child model does not have access to its parent's ``Meta`` class. However, there are a few limited cases where the child inherits behaviour from the parent: if the child does not specify an ``ordering`` attribute or a ``get_latest_by`` attribute, it will inherit these from its parent. If the parent has an ordering and you don't want the child to have any natural ordering, you can explicitly set it to be empty:: class ChildModel(ParentModel): ... class Meta: # Remove parent's ordering effect ordering = [] Inheritance and reverse relations ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Because multi-table inheritance uses an implicit ``OneToOneField`` to link the child and the parent, it's possible to move from the parent down to the child, as in the above example. However, this uses up the name that is the default ``related_name`` value for ``ForeignKey`` and ``ManyToManyField`` relations. If you are putting those type of relations on a subclass of another model, you **must** specify the ``related_name`` attribute on each such field. If you forget, Django will raise an error when you run ``manage.py validate`` or try to syncdb. For example, using the above ``Place`` class again, let's create another subclass with a ``ManyToManyField``:: class Supplier(Place): # Must specify related_name on all relations. customers = models.ManyToManyField(Restaurant, related_name='provider') For more information about reverse relations, refer to the `Database API reference`_ . For now, just remember to run ``manage.py validate`` when you're writing your models and pay attention to the error messages. Specifying the parent link field ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As mentioned, Django will automatically create a ``OneToOneField`` linking your child class back any non-abstract parent models. If you want to control the name of the attribute linking back to the parent, you can create your own link field and pass it ``parent_link=True``. For example, to explicitly specify the field that will link ``Supplier`` to ``Place`` in the above example, you could write:: class Supplier(Place): parent = models.OneToOneField(Place, parent_link=True) ... Multiple inheritance -------------------- Just as with Python's subclassing, it's possible for a Django model to inherit from multiple parent models. Keep in mind that normal Python name resolution rules apply. The first base class that a particular name appears in (e.g. ``Meta``) will be the one that is used. We stop searching once we find the name once. This means that if multiple parents contain a ``Meta`` class, only the first one is going to be used. All others will be ignored. Generally, you won't need to inherit from multiple parents. The main use-case where this is useful is for ''mix-in'' classes: adding a particular extra field or method to every class that inherits the mix-in. Try to keep your inheritance hierarchies as simple and straightforward as possible so that you won't have to struggle to work out where a particular piece of information is coming from. Models across files =================== It's perfectly OK to relate a model to one from another app. To do this, just import the related model at the top of the model that holds your model. Then, just refer to the other model class wherever needed. For example:: from mysite.geography.models import ZipCode class Restaurant(models.Model): # ... zip_code = models.ForeignKey(ZipCode) Using models ============ Once you have created your models, the final step is to tell Django you're going to *use* those models. Do this by editing your settings file and changing the ``INSTALLED_APPS`` setting to add the name of the module that contains your ``models.py``. For example, if the models for your application live in the module ``mysite.myapp.models`` (the package structure that is created for an application by the ``manage.py startapp`` script), ``INSTALLED_APPS`` should read, in part:: INSTALLED_APPS = ( #... 'mysite.myapp', #... ) Providing initial SQL data ========================== Django provides a hook for passing the database arbitrary SQL that's executed just after the CREATE TABLE statements. Use this hook, for example, if you want to populate default records, or create SQL functions, automatically. The hook is simple: Django just looks for a file called ``<appname>/sql/<modelname>.sql``, where ``<appname>`` is your app directory and ``<modelname>`` is the model's name in lowercase. In the ``Person`` example model at the top of this document, assuming it lives in an app called ``myapp``, you could add arbitrary SQL to the file ``myapp/sql/person.sql``. Here's an example of what the file might contain:: INSERT INTO myapp_person (first_name, last_name) VALUES ('John', 'Lennon'); INSERT INTO myapp_person (first_name, last_name) VALUES ('Paul', 'McCartney'); Each SQL file, if given, is expected to contain valid SQL. The SQL files are piped directly into the database after all of the models' table-creation statements have been executed. The SQL files are read by the ``sqlcustom``, ``sqlreset``, ``sqlall`` and ``reset`` commands in ``manage.py``. Refer to the `manage.py documentation`_ for more information. Note that if you have multiple SQL data files, there's no guarantee of the order in which they're executed. The only thing you can assume is that, by the time your custom data files are executed, all the database tables already will have been created. .. _`manage.py documentation`: ../django-admin/#sqlcustom-appname-appname Database-backend-specific SQL data ---------------------------------- There's also a hook for backend-specific SQL data. For example, you can have separate initial-data files for PostgreSQL and MySQL. For each app, Django looks for a file called ``<appname>/sql/<modelname>.<backend>.sql``, where ``<appname>`` is your app directory, ``<modelname>`` is the model's name in lowercase and ``<backend>`` is the value of ``DATABASE_ENGINE`` in your settings file (e.g., ``postgresql``, ``mysql``). Backend-specific SQL data is executed before non-backend-specific SQL data. For example, if your app contains the files ``sql/person.sql`` and ``sql/person.postgresql.sql`` and you're installing the app on PostgreSQL, Django will execute the contents of ``sql/person.postgresql.sql`` first, then ``sql/person.sql``.