562 lines
21 KiB
Plaintext
562 lines
21 KiB
Plaintext
.. _ref-models-instances:
|
|
|
|
========================
|
|
Model instance reference
|
|
========================
|
|
|
|
.. currentmodule:: django.db.models
|
|
|
|
This document describes the details of the ``Model`` API. It builds on the
|
|
material presented in the :ref:`model <topics-db-models>` and :ref:`database
|
|
query <topics-db-queries>` guides, so you'll probably want to read and
|
|
understand those documents before reading this one.
|
|
|
|
Throughout this reference we'll use the :ref:`example weblog models
|
|
<queryset-model-example>` presented in the :ref:`database query guide
|
|
<topics-db-queries>`.
|
|
|
|
Creating objects
|
|
================
|
|
|
|
To create a new instance of a model, just instantiate it like any other Python
|
|
class:
|
|
|
|
.. class:: Model(**kwargs)
|
|
|
|
The keyword arguments are simply the names of the fields you've defined on your
|
|
model. Note that instantiating a model in no way touches your database; for
|
|
that, you need to ``save()``.
|
|
|
|
.. _validating-objects:
|
|
|
|
Validating objects
|
|
==================
|
|
|
|
.. versionadded:: 1.2
|
|
|
|
There are three steps in validating a model, and all three are called by a
|
|
model's ``full_clean()`` method. Most of the time, this method will be called
|
|
automatically by a ``ModelForm``. (See the :ref:`ModelForm documentation
|
|
<topics-forms-modelforms>` for more information.) You should only need to call
|
|
``full_clean()`` if you plan to handle validation errors yourself.
|
|
|
|
.. method:: Model.full_clean(exclude=None)
|
|
|
|
This method calls ``Model.clean_fields()``, ``Model.clean()``, and
|
|
``Model.validate_unique()``, in that order and raises a ``ValidationError``
|
|
that has a ``message_dict`` attribute containing errors from all three stages.
|
|
|
|
The optional ``exclude`` argument can be used to provide a list of field names
|
|
that can be excluded from validation and cleaning. ``ModelForm`` uses this
|
|
argument to exclude fields that aren't present on your form from being
|
|
validated since any errors raised could not be corrected by the user.
|
|
|
|
Note that ``full_clean()`` will NOT be called automatically when you call
|
|
your model's ``save()`` method. You'll need to call it manually if you want
|
|
to run model validation outside of a ``ModelForm``. (This is for backwards
|
|
compatibility.)
|
|
|
|
Example::
|
|
|
|
try:
|
|
article.full_clean()
|
|
except ValidationError, e:
|
|
# Do something based on the errors contained in e.error_dict.
|
|
# Display them to a user, or handle them programatically.
|
|
|
|
The first step ``full_clean()`` performs is to clean each individual field.
|
|
|
|
.. method:: Model.clean_fields(exclude=None)
|
|
|
|
This method will validate all fields on your model. The optional ``exclude``
|
|
argument lets you provide a list of field names to exclude from validation. It
|
|
will raise a ``ValidationError`` if any fields fail validation.
|
|
|
|
The second step ``full_clean()`` performs is to call ``Model.clean()``.
|
|
This method should be overridden to perform custom validation on your model.
|
|
|
|
.. method:: Model.clean()
|
|
|
|
This method should be used to provide custom model validation, and to modify
|
|
attributes on your model if desired. For instance, you could use it to
|
|
automatically provide a value for a field, or to do validation that requires
|
|
access to more than a single field::
|
|
|
|
def clean(self):
|
|
from django.core.exceptions import ValidationError
|
|
# Don't allow draft entries to have a pub_date.
|
|
if self.status == 'draft' and self.pub_date is not None:
|
|
raise ValidationError('Draft entries may not have a publication date.')
|
|
# Set the pub_date for published items if it hasn't been set already.
|
|
if self.status == 'published' and self.pub_date is None:
|
|
self.pub_date = datetime.datetime.now()
|
|
|
|
Any ``ValidationError`` raised by ``Model.clean()`` will be stored under a
|
|
special key that is used for errors that are tied to the entire model instead
|
|
of to a specific field. You can access these errors with ``NON_FIELD_ERRORS``::
|
|
|
|
|
|
from django.core.validators import ValidationError, NON_FIELD_ERRORS
|
|
try:
|
|
article.full_clean():
|
|
except ValidationError, e:
|
|
non_field_errors = e.message_dict[NON_FIELD_ERRORS]
|
|
|
|
Finally, ``full_clean()`` will check any unique constraints on your model.
|
|
|
|
.. method:: Model.validate_unique(exclude=None)
|
|
|
|
This method is similar to ``clean_fields``, but validates all uniqueness
|
|
constraints on your model instead of individual field values. The optional
|
|
``exclude`` argument allows you to provide a list of field names to exclude
|
|
from validation. It will raise a ``ValidationError`` if any fields fail
|
|
validation.
|
|
|
|
Note that if you provide an ``exclude`` argument to ``validate_unique``, any
|
|
``unique_together`` constraint that contains one of the fields you provided
|
|
will not be checked.
|
|
|
|
|
|
Saving objects
|
|
==============
|
|
|
|
To save an object back to the database, call ``save()``:
|
|
|
|
.. method:: Model.save([force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS])
|
|
|
|
.. versionadded:: 1.0
|
|
The ``force_insert`` and ``force_update`` arguments were added.
|
|
|
|
.. versionadded:: 1.2
|
|
The ``using`` argument was added.
|
|
|
|
If you want customized saving behavior, you can override this
|
|
``save()`` method. See :ref:`overriding-model-methods` for more
|
|
details.
|
|
|
|
The model save process also has some subtleties; see the sections
|
|
below.
|
|
|
|
Auto-incrementing primary keys
|
|
------------------------------
|
|
|
|
If a model has an ``AutoField`` -- an auto-incrementing primary key -- then
|
|
that auto-incremented value will be calculated and saved as an attribute on
|
|
your object the first time you call ``save()``::
|
|
|
|
>>> b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')
|
|
>>> b2.id # Returns None, because b doesn't have an ID yet.
|
|
>>> b2.save()
|
|
>>> b2.id # Returns the ID of your new object.
|
|
|
|
There's no way to tell what the value of an ID will be before you call
|
|
``save()``, because that value is calculated by your database, not by Django.
|
|
|
|
(For convenience, each model has an ``AutoField`` named ``id`` by default
|
|
unless you explicitly specify ``primary_key=True`` on a field. See the
|
|
documentation for ``AutoField`` for more details.
|
|
|
|
The ``pk`` property
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. versionadded:: 1.0
|
|
|
|
.. attribute:: Model.pk
|
|
|
|
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.
|
|
|
|
Explicitly specifying auto-primary-key values
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
If a model has an ``AutoField`` but you want to define a new object's ID
|
|
explicitly when saving, just define it explicitly before saving, rather than
|
|
relying on the auto-assignment of the ID::
|
|
|
|
>>> b3 = Blog(id=3, name='Cheddar Talk', tagline='Thoughts on cheese.')
|
|
>>> b3.id # Returns 3.
|
|
>>> b3.save()
|
|
>>> b3.id # Returns 3.
|
|
|
|
If you assign auto-primary-key values manually, make sure not to use an
|
|
already-existing primary-key value! If you create a new object with an explicit
|
|
primary-key value that already exists in the database, Django will assume you're
|
|
changing the existing record rather than creating a new one.
|
|
|
|
Given the above ``'Cheddar Talk'`` blog example, this example would override the
|
|
previous record in the database::
|
|
|
|
b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')
|
|
b4.save() # Overrides the previous blog with ID=3!
|
|
|
|
See `How Django knows to UPDATE vs. INSERT`_, below, for the reason this
|
|
happens.
|
|
|
|
Explicitly specifying auto-primary-key values is mostly useful for bulk-saving
|
|
objects, when you're confident you won't have primary-key collision.
|
|
|
|
What happens when you save?
|
|
---------------------------
|
|
|
|
When you save an object, Django performs the following steps:
|
|
|
|
1. **Emit a pre-save signal.** The :ref:`signal <ref-signals>`
|
|
:attr:`django.db.models.signals.pre_save` is sent, allowing any
|
|
functions listening for that signal to take some customized
|
|
action.
|
|
|
|
2. **Pre-process the data.** Each field on the object is asked to
|
|
perform any automated data modification that the field may need
|
|
to perform.
|
|
|
|
Most fields do *no* pre-processing -- the field data is kept as-is.
|
|
Pre-processing is only used on fields that have special behavior.
|
|
For example, if your model has a ``DateField`` with ``auto_now=True``,
|
|
the pre-save phase will alter the data in the object to ensure that
|
|
the date field contains the current date stamp. (Our documentation
|
|
doesn't yet include a list of all the fields with this "special
|
|
behavior.")
|
|
|
|
3. **Prepare the data for the database.** Each field is asked to provide
|
|
its current value in a data type that can be written to the database.
|
|
|
|
Most fields require *no* data preparation. Simple data types, such as
|
|
integers and strings, are 'ready to write' as a Python object. However,
|
|
more complex data types often require some modification.
|
|
|
|
For example, ``DateFields`` use a Python ``datetime`` object to store
|
|
data. Databases don't store ``datetime`` objects, so the field value
|
|
must be converted into an ISO-compliant date string for insertion
|
|
into the database.
|
|
|
|
4. **Insert the data into the database.** The pre-processed, prepared
|
|
data is then composed into an SQL statement for insertion into the
|
|
database.
|
|
|
|
5. **Emit a post-save signal.** The signal
|
|
:attr:`django.db.models.signals.post_save` is sent, allowing
|
|
any functions listening for that signal to take some customized
|
|
action.
|
|
|
|
How Django knows to UPDATE vs. INSERT
|
|
-------------------------------------
|
|
|
|
You may have noticed Django database objects use the same ``save()`` method
|
|
for creating and changing objects. Django abstracts the need to use ``INSERT``
|
|
or ``UPDATE`` SQL statements. Specifically, when you call ``save()``, Django
|
|
follows this algorithm:
|
|
|
|
* If the object's primary key attribute is set to a value that evaluates to
|
|
``True`` (i.e., a value other than ``None`` or the empty string), Django
|
|
executes a ``SELECT`` query to determine whether a record with the given
|
|
primary key already exists.
|
|
* If the record with the given primary key does already exist, Django
|
|
executes an ``UPDATE`` query.
|
|
* If the object's primary key attribute is *not* set, or if it's set but a
|
|
record doesn't exist, Django executes an ``INSERT``.
|
|
|
|
The one gotcha here is that you should be careful not to specify a primary-key
|
|
value explicitly when saving new objects, if you cannot guarantee the
|
|
primary-key value is unused. For more on this nuance, see `Explicitly specifying
|
|
auto-primary-key values`_ above and `Forcing an INSERT or UPDATE`_ below.
|
|
|
|
.. _ref-models-force-insert:
|
|
|
|
Forcing an INSERT or UPDATE
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. versionadded:: 1.0
|
|
|
|
In some rare circumstances, it's necessary to be able to force the ``save()``
|
|
method to perform an SQL ``INSERT`` and not fall back to doing an ``UPDATE``.
|
|
Or vice-versa: update, if possible, but not insert a new row. In these cases
|
|
you can pass the ``force_insert=True`` or ``force_update=True`` parameters to
|
|
the ``save()`` method. Passing both parameters is an error, since you cannot
|
|
both insert *and* update at the same time.
|
|
|
|
It should be very rare that you'll need to use these parameters. Django will
|
|
almost always do the right thing and trying to override that will lead to
|
|
errors that are difficult to track down. This feature is for advanced use
|
|
only.
|
|
|
|
Updating attributes based on existing fields
|
|
--------------------------------------------
|
|
|
|
Sometimes you'll need to perform a simple arithmetic task on a field, such
|
|
as incrementing or decrementing the current value. The obvious way to
|
|
achieve this is to do something like::
|
|
|
|
>>> product = Product.objects.get(name='Venezuelan Beaver Cheese')
|
|
>>> product.number_sold += 1
|
|
>>> product.save()
|
|
|
|
If the old ``number_sold`` value retrieved from the database was 10, then
|
|
the value of 11 will be written back to the database.
|
|
|
|
This can be optimized slightly by expressing the update relative to the
|
|
original field value, rather than as an explicit assignment of a new value.
|
|
Django provides :ref:`F() expressions <query-expressions>` as a way of
|
|
performing this kind of relative update. Using ``F()`` expressions, the
|
|
previous example would be expressed as::
|
|
|
|
>>> from django.db.models import F
|
|
>>> product = Product.objects.get(name='Venezuelan Beaver Cheese')
|
|
>>> product.number_sold = F('number_sold') + 1
|
|
>>> product.save()
|
|
|
|
This approach doesn't use the initial value from the database. Instead, it
|
|
makes the database do the update based on whatever value is current at the
|
|
time that the save() is executed.
|
|
|
|
Once the object has been saved, you must reload the object in order to access
|
|
the actual value that was applied to the updated field::
|
|
|
|
>>> product = Products.objects.get(pk=product.pk)
|
|
>>> print product.number_sold
|
|
42
|
|
|
|
For more details, see the documentation on :ref:`F() expressions
|
|
<query-expressions>` and their :ref:`use in update queries
|
|
<topics-db-queries-update>`.
|
|
|
|
Deleting objects
|
|
================
|
|
|
|
.. method:: Model.delete([using=DEFAULT_DB_ALIAS])
|
|
|
|
.. versionadded:: 1.2
|
|
The ``using`` argument was added.
|
|
|
|
Issues a SQL ``DELETE`` for the object. This only deletes the object
|
|
in the database; the Python instance will still be around, and will
|
|
still have data in its fields.
|
|
|
|
For more details, including how to delete objects in bulk, see
|
|
:ref:`topics-db-queries-delete`.
|
|
|
|
If you want customized deletion behavior, you can override this
|
|
``delete()`` method. See :ref:`overriding-model-methods` for more
|
|
details.
|
|
|
|
.. _model-instance-methods:
|
|
|
|
Other model instance methods
|
|
============================
|
|
|
|
A few object methods have special purposes.
|
|
|
|
``__str__``
|
|
-----------
|
|
|
|
.. method:: Model.__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__``
|
|
---------------
|
|
|
|
.. method:: Model.__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``
|
|
--------------------
|
|
|
|
.. method:: Model.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 :ref:`syndication feed
|
|
framework <ref-contrib-syndication>`, use ``get_absolute_url()`` as a
|
|
convenience to reward people who've defined the method.
|
|
|
|
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:
|
|
|
|
.. function:: permalink()
|
|
|
|
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+)/$', 'people.views.details'),
|
|
|
|
...your model could have a ``get_absolute_url`` method that looked like this::
|
|
|
|
from django.db import models
|
|
|
|
@models.permalink
|
|
def get_absolute_url(self):
|
|
return ('people.views.details', [str(self.id)])
|
|
|
|
Similarly, if you had a URLconf entry that looked like::
|
|
|
|
(r'/archive/(?P<year>\d{4})/(?P<month>\d{1,2})/(?P<day>\d{1,2})/$', archive_view)
|
|
|
|
...you could reference this using ``permalink()`` as follows::
|
|
|
|
@models.permalink
|
|
def get_absolute_url(self):
|
|
return ('archive_view', (), {
|
|
'year': self.created.year,
|
|
'month': self.created.month,
|
|
'day': self.created.day})
|
|
|
|
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
|
|
|
|
@models.permalink
|
|
def get_absolute_url(self):
|
|
return ('people_view', [str(self.id)])
|
|
|
|
More details on named URL patterns are in the :ref:`URL dispatch documentation
|
|
<topics-http-urls>`.
|
|
|
|
Extra instance methods
|
|
======================
|
|
|
|
In addition to ``save()``, ``delete()``, a model object might get any or all
|
|
of the following methods:
|
|
|
|
.. method:: Model.get_FOO_display()
|
|
|
|
For every field that has ``choices`` set, the object will have a
|
|
``get_FOO_display()`` method, where ``FOO`` is the name of the field. This
|
|
method returns the "human-readable" value of the field. For example, in the
|
|
following model::
|
|
|
|
GENDER_CHOICES = (
|
|
('M', 'Male'),
|
|
('F', 'Female'),
|
|
)
|
|
class Person(models.Model):
|
|
name = models.CharField(max_length=20)
|
|
gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
|
|
|
|
...each ``Person`` instance will have a ``get_gender_display()`` method. Example::
|
|
|
|
>>> p = Person(name='John', gender='M')
|
|
>>> p.save()
|
|
>>> p.gender
|
|
'M'
|
|
>>> p.get_gender_display()
|
|
'Male'
|
|
|
|
.. method:: Model.get_next_by_FOO(\**kwargs)
|
|
.. method:: Model.get_previous_by_FOO(\**kwargs)
|
|
|
|
For every ``DateField`` and ``DateTimeField`` that does not have ``null=True``,
|
|
the object will have ``get_next_by_FOO()`` and ``get_previous_by_FOO()``
|
|
methods, where ``FOO`` is the name of the field. This returns the next and
|
|
previous object with respect to the date field, raising the appropriate
|
|
``DoesNotExist`` exception when appropriate.
|
|
|
|
Both methods accept optional keyword arguments, which should be in the format
|
|
described in :ref:`Field lookups <field-lookups>`.
|
|
|
|
Note that in the case of identical date values, these methods will use the ID
|
|
as a fallback check. This guarantees that no records are skipped or duplicated.
|