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179 lines
7 KiB
Text
179 lines
7 KiB
Text
=========================
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Related objects reference
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=========================
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.. currentmodule:: django.db.models.fields.related
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.. class:: RelatedManager
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A "related manager" is a manager used in a one-to-many or many-to-many
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related context. This happens in two cases:
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* The "other side" of a :class:`~django.db.models.ForeignKey` relation.
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That is::
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from django.db import models
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class Reporter(models.Model):
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# ...
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pass
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class Article(models.Model):
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reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
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In the above example, the methods below will be available on
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the manager ``reporter.article_set``.
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* Both sides of a :class:`~django.db.models.ManyToManyField` relation::
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class Topping(models.Model):
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# ...
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pass
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class Pizza(models.Model):
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toppings = models.ManyToManyField(Topping)
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In this example, the methods below will be available both on
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``topping.pizza_set`` and on ``pizza.toppings``.
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.. method:: add(*objs, bulk=True)
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Adds the specified model objects to the related object set.
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Example::
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>>> b = Blog.objects.get(id=1)
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>>> e = Entry.objects.get(id=234)
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>>> b.entry_set.add(e) # Associates Entry e with Blog b.
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In the example above, in the case of a
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:class:`~django.db.models.ForeignKey` relationship,
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:meth:`QuerySet.update() <django.db.models.query.QuerySet.update>`
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is used to perform the update. This requires the objects to already be
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saved.
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You can use the ``bulk=False`` argument to instead have the related
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manager perform the update by calling ``e.save()``.
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Using ``add()`` with a many-to-many relationship, however, will not
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call any ``save()`` methods, but rather create the relationships
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using :meth:`QuerySet.bulk_create()
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<django.db.models.query.QuerySet.bulk_create>`. If you need to execute
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some custom logic when a relationship is created, listen to the
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:data:`~django.db.models.signals.m2m_changed` signal.
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.. method:: create(**kwargs)
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Creates a new object, saves it and puts it in the related object set.
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Returns the newly created object::
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>>> b = Blog.objects.get(id=1)
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>>> e = b.entry_set.create(
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... headline='Hello',
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... body_text='Hi',
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... pub_date=datetime.date(2005, 1, 1)
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... )
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# No need to call e.save() at this point -- it's already been saved.
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This is equivalent to (but much simpler than)::
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>>> b = Blog.objects.get(id=1)
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>>> e = Entry(
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... blog=b,
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... headline='Hello',
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... body_text='Hi',
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... pub_date=datetime.date(2005, 1, 1)
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... )
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>>> e.save(force_insert=True)
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Note that there's no need to specify the keyword argument of the model
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that defines the relationship. In the above example, we don't pass the
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parameter ``blog`` to ``create()``. Django figures out that the new
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``Entry`` object's ``blog`` field should be set to ``b``.
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.. method:: remove(*objs)
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Removes the specified model objects from the related object set::
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>>> b = Blog.objects.get(id=1)
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>>> e = Entry.objects.get(id=234)
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>>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.
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Similar to :meth:`add()`, ``e.save()`` is called in the example above
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to perform the update. Using ``remove()`` with a many-to-many
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relationship, however, will delete the relationships using
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:meth:`QuerySet.delete()<django.db.models.query.QuerySet.delete>` which
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means no model ``save()`` methods are called; listen to the
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:data:`~django.db.models.signals.m2m_changed` signal if you wish to
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execute custom code when a relationship is deleted.
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For :class:`~django.db.models.ForeignKey` objects, this method only
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exists if ``null=True``. If the related field can't be set to ``None``
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(``NULL``), then an object can't be removed from a relation without
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being added to another. In the above example, removing ``e`` from
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``b.entry_set()`` is equivalent to doing ``e.blog = None``, and because
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the ``blog`` :class:`~django.db.models.ForeignKey` doesn't have
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``null=True``, this is invalid.
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For :class:`~django.db.models.ForeignKey` objects, this method accepts
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a ``bulk`` argument to control how to perform the operation.
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If ``True`` (the default), ``QuerySet.update()`` is used.
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If ``bulk=False``, the ``save()`` method of each individual model
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instance is called instead. This triggers the
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:data:`~django.db.models.signals.pre_save` and
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:data:`~django.db.models.signals.post_save` signals and comes at the
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expense of performance.
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.. method:: clear()
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Removes all objects from the related object set::
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>>> b = Blog.objects.get(id=1)
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>>> b.entry_set.clear()
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Note this doesn't delete the related objects -- it just disassociates
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them.
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Just like ``remove()``, ``clear()`` is only available on
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:class:`~django.db.models.ForeignKey`\s where ``null=True`` and it also
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accepts the ``bulk`` keyword argument.
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.. method:: set(objs, bulk=True, clear=False)
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Replace the set of related objects::
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>>> new_list = [obj1, obj2, obj3]
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>>> e.related_set.set(new_list)
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This method accepts a ``clear`` argument to control how to perform the
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operation. If ``False`` (the default), the elements missing from the
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new set are removed using ``remove()`` and only the new ones are added.
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If ``clear=True``, the ``clear()`` method is called instead and the
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whole set is added at once.
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The ``bulk`` argument is passed on to :meth:`add`.
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Note that since ``set()`` is a compound operation, it is subject to
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race conditions. For instance, new objects may be added to the database
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in between the call to ``clear()`` and the call to ``add()``.
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.. note::
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Note that ``add()``, ``create()``, ``remove()``, ``clear()``, and
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``set()`` all apply database changes immediately for all types of
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related fields. In other words, there is no need to call ``save()``
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on either end of the relationship.
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Also, if you are using :ref:`an intermediate model
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<intermediary-manytomany>` for a many-to-many relationship, then the
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``add()``, ``create()``, ``remove()``, and ``set()`` methods are
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disabled.
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If you use :meth:`~django.db.models.query.QuerySet.prefetch_related`,
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the ``add()``, ``remove()``, ``clear()``, and ``set()`` methods clear
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the prefetched cache.
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.. versionchanged:: 1.11
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The clearing of the prefetched cache described above was added.
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