682 lines
27 KiB
Plaintext
682 lines
27 KiB
Plaintext
========================
|
|
Model instance reference
|
|
========================
|
|
|
|
.. currentmodule:: django.db.models
|
|
|
|
This document describes the details of the ``Model`` API. It builds on the
|
|
material presented in the :doc:`model </topics/db/models>` and :doc:`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 :doc:`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 :meth:`~Model.save()`.
|
|
|
|
.. note::
|
|
|
|
You may be tempted to customize the model by overriding the ``__init__``
|
|
method. If you do so, however, take care not to change the calling
|
|
signature as any change may prevent the model instance from being saved.
|
|
Rather than overriding ``__init__``, try using one of these approaches:
|
|
|
|
1. Add a classmethod on the model class::
|
|
|
|
from django.db import models
|
|
|
|
class Book(models.Model):
|
|
title = models.CharField(max_length=100)
|
|
|
|
@classmethod
|
|
def create(cls, title):
|
|
book = cls(title=title)
|
|
# do something with the book
|
|
return book
|
|
|
|
book = Book.create("Pride and Prejudice")
|
|
|
|
2. Add a method on a custom manager (usually preferred)::
|
|
|
|
class BookManager(models.Manager):
|
|
def create_book(self, title):
|
|
book = self.create(title=title)
|
|
# do something with the book
|
|
return book
|
|
|
|
class Book(models.Model):
|
|
title = models.CharField(max_length=100)
|
|
|
|
objects = BookManager()
|
|
|
|
book = Book.objects.create_book("Pride and Prejudice")
|
|
|
|
.. _validating-objects:
|
|
|
|
Validating objects
|
|
==================
|
|
|
|
There are three steps involved in validating a model:
|
|
|
|
1. Validate the model fields - :meth:`Model.clean_fields()`
|
|
2. Validate the model as a whole - :meth:`Model.clean()`
|
|
3. Validate the field uniqueness - :meth:`Model.validate_unique()`
|
|
|
|
All three steps are performed when you call a model's
|
|
:meth:`~Model.full_clean()` method.
|
|
|
|
When you use a :class:`~django.forms.ModelForm`, the call to
|
|
:meth:`~django.forms.Form.is_valid()` will perform these validation steps for
|
|
all the fields that are included on the form. See the :doc:`ModelForm
|
|
documentation </topics/forms/modelforms>` for more information. You should only
|
|
need to call a model's :meth:`~Model.full_clean()` method if you plan to handle
|
|
validation errors yourself, or if you have excluded fields from the
|
|
:class:`~django.forms.ModelForm` that require validation.
|
|
|
|
.. method:: Model.full_clean(exclude=None)
|
|
|
|
This method calls :meth:`Model.clean_fields()`, :meth:`Model.clean()`, and
|
|
:meth:`Model.validate_unique()`, in that order and raises a
|
|
:exc:`~django.core.exceptions.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.
|
|
:class:`~django.forms.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 :meth:`~Model.save()` method, nor as a result of
|
|
:class:`~django.forms.ModelForm` validation. In the case of
|
|
:class:`~django.forms.ModelForm` validation, :meth:`Model.clean_fields()`,
|
|
:meth:`Model.clean()`, and :meth:`Model.validate_unique()` are all called
|
|
individually.
|
|
|
|
You'll need to call ``full_clean`` manually when you want to run one-step model
|
|
validation for your own manually created models. For example::
|
|
|
|
from django.core.exceptions import ValidationError
|
|
try:
|
|
article.full_clean()
|
|
except ValidationError as e:
|
|
# Do something based on the errors contained in e.message_dict.
|
|
# Display them to a user, or handle them programatically.
|
|
pass
|
|
|
|
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 :exc:`~django.core.exceptions.ValidationError` if any fields fail
|
|
validation.
|
|
|
|
The second step ``full_clean()`` performs is to call :meth:`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):
|
|
import datetime
|
|
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.date.today()
|
|
|
|
Any :exc:`~django.core.exceptions.ValidationError` exceptions raised by
|
|
``Model.clean()`` will be stored in a special key error dictionary key,
|
|
``NON_FIELD_ERRORS``, that is used for errors that are tied to the entire model
|
|
instead of to a specific field::
|
|
|
|
from django.core.exceptions import ValidationError, NON_FIELD_ERRORS
|
|
try:
|
|
article.full_clean()
|
|
except ValidationError as 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 :meth:`~Model.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
|
|
:exc:`~django.core.exceptions.ValidationError` if any fields fail validation.
|
|
|
|
Note that if you provide an ``exclude`` argument to ``validate_unique()``, any
|
|
:attr:`~django.db.models.Options.unique_together` constraint involving 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, update_fields=None])
|
|
|
|
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 :class:`~django.db.models.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 :class:`~django.db.models.AutoField` named
|
|
``id`` by default unless you explicitly specify ``primary_key=True`` on a field
|
|
in your model. See the documentation for :class:`~django.db.models.AutoField`
|
|
for more details.
|
|
|
|
The ``pk`` property
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. 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 :class:`~django.db.models.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 :doc:`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 :class:`~django.db.models.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, :class:`~django.db.models.DateField` fields 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 an ``UPDATE``.
|
|
* If the object's primary key attribute is *not* set or if the ``UPDATE``
|
|
didn't update anything, Django executes an ``INSERT``.
|
|
|
|
.. versionchanged:: 1.6
|
|
|
|
Previously Django used ``SELECT`` - if not found ``INSERT`` else ``UPDATE``
|
|
algorithm. The old algorithm resulted in one more query in ``UPDATE`` case.
|
|
|
|
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
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
In some rare circumstances, it's necessary to be able to force the
|
|
:meth:`~Model.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 :meth:`~Model.save()` method.
|
|
Obviously, passing both parameters is an error: 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.
|
|
|
|
Using ``update_fields`` will force an update similarly to ``force_update``.
|
|
|
|
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 sequence has a standard update problem in that it contains a race
|
|
condition. If another thread of execution has already saved an updated value
|
|
after the current thread retrieved the old value, the current thread will only
|
|
save the old value plus one, rather than the new (current) value plus one.
|
|
|
|
The process can be made robust and slightly faster 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>` for
|
|
performing this kind of relative update. Using ``F()`` expressions, the
|
|
previous example is 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 :meth:`~Model.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>`.
|
|
|
|
Specifying which fields to save
|
|
-------------------------------
|
|
|
|
.. versionadded:: 1.5
|
|
|
|
If ``save()`` is passed a list of field names in keyword argument
|
|
``update_fields``, only the fields named in that list will be updated.
|
|
This may be desirable if you want to update just one or a few fields on
|
|
an object. There will be a slight performance benefit from preventing
|
|
all of the model fields from being updated in the database. For example::
|
|
|
|
product.name = 'Name changed again'
|
|
product.save(update_fields=['name'])
|
|
|
|
The ``update_fields`` argument can be any iterable containing strings. An
|
|
empty ``update_fields`` iterable will skip the save. A value of None will
|
|
perform an update on all fields.
|
|
|
|
Specifying ``update_fields`` will force an update.
|
|
|
|
When saving a model fetched through deferred model loading
|
|
(:meth:`~django.db.models.query.QuerySet.only()` or
|
|
:meth:`~django.db.models.query.QuerySet.defer()`) only the fields loaded
|
|
from the DB will get updated. In effect there is an automatic
|
|
``update_fields`` in this case. If you assign or change any deferred field
|
|
value, the field will be added to the updated fields.
|
|
|
|
Deleting objects
|
|
================
|
|
|
|
.. method:: Model.delete([using=DEFAULT_DB_ALIAS])
|
|
|
|
Issues a SQL ``DELETE`` for the object. This only deletes the object in the
|
|
database; the Python instance will still exist 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 the ``delete()``
|
|
method. See :ref:`overriding-model-methods` for more details.
|
|
|
|
.. _model-instance-methods:
|
|
|
|
Other model instance methods
|
|
============================
|
|
|
|
A few object methods have special purposes.
|
|
|
|
``__unicode__``
|
|
---------------
|
|
|
|
.. method:: Model.__unicode__()
|
|
|
|
The ``__unicode__()`` method is called whenever you call ``unicode()`` on an
|
|
object. Django uses ``unicode(obj)`` (or the related function, :meth:`str(obj)
|
|
<Model.__str__>`) in a number of places. Most notably, to display 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
|
|
representation of the model from the ``__unicode__()`` method.
|
|
|
|
For example::
|
|
|
|
from django.db import models
|
|
|
|
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
|
|
:meth:`~Model.__str__()` method, Django will automatically provide you with a
|
|
:meth:`~Model.__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.
|
|
|
|
``__str__``
|
|
-----------
|
|
|
|
.. method:: Model.__str__()
|
|
|
|
The ``__str__()`` method is called whenever you call ``str()`` on an object. The main use for this method directly inside Django is when the ``repr()`` output of a model is displayed anywhere (for example, in debugging output).
|
|
Thus, you should return a nice, human-readable string for the object's
|
|
``__str__()``. It isn't required to put ``__str__()`` methods everywhere if you have sensible :meth:`~Model.__unicode__()` methods.
|
|
|
|
The previous :meth:`~Model.__unicode__()` example could be similarly written
|
|
using ``__str__()`` like this::
|
|
|
|
from django.db import models
|
|
from django.utils.encoding import force_bytes
|
|
|
|
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.force_bytes() here because
|
|
# first_name and last_name will be unicode strings.
|
|
return force_bytes('%s %s' % (self.first_name, self.last_name))
|
|
|
|
``get_absolute_url``
|
|
--------------------
|
|
|
|
.. method:: Model.get_absolute_url()
|
|
|
|
Define a ``get_absolute_url()`` method to tell Django how to calculate the
|
|
canonical URL for an object. To callers, this method should appear to return a
|
|
string that can be used to refer to the object over HTTP.
|
|
|
|
For example::
|
|
|
|
def get_absolute_url(self):
|
|
return "/people/%i/" % self.id
|
|
|
|
(Whilst this code is correct and simple, it may not be the most portable way to
|
|
write this kind of method. The :func:`~django.core.urlresolvers.reverse`
|
|
function is usually the best approach.)
|
|
|
|
For example::
|
|
|
|
def get_absolute_url(self):
|
|
from django.core.urlresolvers import reverse
|
|
return reverse('people.views.details', args=[str(self.id)])
|
|
|
|
One place Django uses ``get_absolute_url()`` is in the admin app. If an object
|
|
defines this method, the object-editing page will have a "View on site" link
|
|
that will jump you directly to the object's public view, as given by
|
|
``get_absolute_url()``.
|
|
|
|
Similarly, a couple of other bits of Django, such as the :doc:`syndication feed
|
|
framework </ref/contrib/syndication>`, use ``get_absolute_url()`` when it is
|
|
defined. If it makes sense for your model's instances to each have a unique
|
|
URL, you should define ``get_absolute_url()``.
|
|
|
|
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:
|
|
|
|
.. code-block:: html+django
|
|
|
|
<!-- BAD template code. Avoid! -->
|
|
<a href="/people/{{ object.id }}/">{{ object.name }}</a>
|
|
|
|
This template code is much better:
|
|
|
|
.. code-block:: html+django
|
|
|
|
<a href="{{ object.get_absolute_url }}">{{ object.name }}</a>
|
|
|
|
The logic here is that if you change the URL structure of your objects, even
|
|
for something simple such as correcting a spelling error, you don't want to
|
|
have to track down every place that the URL might be created. Specify it once,
|
|
in ``get_absolute_url()`` and have all your other code call that one place.
|
|
|
|
.. note::
|
|
The string you return from ``get_absolute_url()`` **must** contain only
|
|
ASCII characters (required by the URI specfication, :rfc:`2396`) and be
|
|
URL-encoded, if necessary.
|
|
|
|
Code and templates calling ``get_absolute_url()`` should be able to use the
|
|
result directly without 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 containing characters outside the ASCII range at
|
|
all.
|
|
|
|
The ``permalink`` decorator
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. warning::
|
|
|
|
The ``permalink`` decorator is no longer recommended. You should use
|
|
:func:`~django.core.urlresolvers.reverse` in the body of your
|
|
``get_absolute_url`` method instead.
|
|
|
|
In early versions of Django, there wasn't an easy way to use URLs defined in
|
|
URLconf file inside :meth:`~django.db.models.Model.get_absolute_url`. That
|
|
meant you would need to define the URL both in URLConf and
|
|
:meth:`~django.db.models.Model.get_absolute_url`. The ``permalink`` decorator
|
|
was added to overcome this DRY principle violation. However, since the
|
|
introduction of :func:`~django.core.urlresolvers.reverse` there is no
|
|
reason to use ``permalink`` any more.
|
|
|
|
.. function:: permalink()
|
|
|
|
This decorator takes the name of a URL pattern (either a view name or a URL
|
|
pattern name) and a list of position or keyword arguments and uses the URLconf
|
|
patterns to construct the correct, full URL. It returns a string for the
|
|
correct URL, with all parameters substituted in the correct positions.
|
|
|
|
The ``permalink`` decorator is a Python-level equivalent to the :ttag:`url`
|
|
template tag and a high-level wrapper for the
|
|
:func:`~django.core.urlresolvers.reverse` function.
|
|
|
|
An example should make it clear how to use ``permalink()``. Suppose your URLconf
|
|
contains a line such as::
|
|
|
|
(r'^people/(\d+)/$', 'people.views.details'),
|
|
|
|
...your model could have a :meth:`~django.db.models.Model.get_absolute_url`
|
|
method that looked like this::
|
|
|
|
from django.db import models
|
|
|
|
@models.permalink
|
|
def get_absolute_url(self):
|
|
return ('people.views.details', [str(self.id)])
|
|
|
|
Similarly, if you had a URLconf entry that looked like::
|
|
|
|
(r'/archive/(?P<year>\d{4})/(?P<month>\d{2})/(?P<day>\d{2})/$', 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.strftime('%m'),
|
|
'day': self.created.strftime('%d')})
|
|
|
|
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 associating the model's absolute path with the view that is
|
|
used to display it, without repeating the view's URL information anywhere. You
|
|
can still use the :meth:`~django.db.models.Model.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 case,
|
|
Django has :ref:`named URL patterns <naming-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 import url
|
|
|
|
url(r'^people/(\d+)/$', 'blog_views.generic_detail', name='people_view'),
|
|
|
|
...and then using that name to perform the reverse URL resolution instead
|
|
of the view name::
|
|
|
|
from django.db import models
|
|
|
|
@models.permalink
|
|
def get_absolute_url(self):
|
|
return ('people_view', [str(self.id)])
|
|
|
|
More details on named URL patterns are in the :doc:`URL dispatch documentation
|
|
</topics/http/urls>`.
|
|
|
|
Extra instance methods
|
|
======================
|
|
|
|
In addition to :meth:`~Model.save()`, :meth:`~Model.delete()`, a model object
|
|
might have some of the following methods:
|
|
|
|
.. method:: Model.get_FOO_display()
|
|
|
|
For every field that has :attr:`~django.db.models.Field.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::
|
|
|
|
from django.db import models
|
|
|
|
class Person(models.Model):
|
|
SHIRT_SIZES = (
|
|
(u'S', u'Small'),
|
|
(u'M', u'Medium'),
|
|
(u'L', u'Large'),
|
|
)
|
|
name = models.CharField(max_length=60)
|
|
shirt_size = models.CharField(max_length=2, choices=SHIRT_SIZES)
|
|
|
|
::
|
|
|
|
>>> p = Person(name="Fred Flintstone", shirt_size="L")
|
|
>>> p.save()
|
|
>>> p.shirt_size
|
|
u'L'
|
|
>>> p.get_shirt_size_display()
|
|
u'Large'
|
|
|
|
.. method:: Model.get_next_by_FOO(\**kwargs)
|
|
.. method:: Model.get_previous_by_FOO(\**kwargs)
|
|
|
|
For every :class:`~django.db.models.DateField` and
|
|
:class:`~django.db.models.DateTimeField` that does not have :attr:`null=True
|
|
<django.db.models.Field.null>`, 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
|
|
a :exc:`~django.core.exceptions.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
|
|
primary key as a tie-breaker. This guarantees that no records are skipped or
|
|
duplicated. That also means you cannot use those methods on unsaved objects.
|