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=========================
Related objects reference
=========================
.. currentmodule:: django.db.models.fields.related
.. class:: RelatedManager
A "related manager" is a manager used in a one-to-many or many-to-many
related context. This happens in two cases:
* The "other side" of a :class:`~django.db.models.ForeignKey` relation.
That is::
from django.db import models
class Reporter(models.Model):
# ...
pass
class Article(models.Model):
reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
In the above example, the methods below will be available on
the manager ``reporter.article_set``.
* Both sides of a :class:`~django.db.models.ManyToManyField` relation::
class Topping(models.Model):
# ...
pass
class Pizza(models.Model):
toppings = models.ManyToManyField(Topping)
In this example, the methods below will be available both on
``topping.pizza_set`` and on ``pizza.toppings``.
.. method:: add(*objs, bulk=True)
Adds the specified model objects to the related object set.
Example::
>>> b = Blog.objects.get(id=1)
>>> e = Entry.objects.get(id=234)
>>> b.entry_set.add(e) # Associates Entry e with Blog b.
In the example above, in the case of a
:class:`~django.db.models.ForeignKey` relationship,
:meth:`QuerySet.update() <django.db.models.query.QuerySet.update>`
is used to perform the update. This requires the objects to already be
saved.
You can use the ``bulk=False`` argument to instead have the related
manager perform the update by calling ``e.save()``.
Using ``add()`` with a many-to-many relationship, however, will not
call any ``save()`` methods, but rather create the relationships
using :meth:`QuerySet.bulk_create()
<django.db.models.query.QuerySet.bulk_create>`. If you need to execute
some custom logic when a relationship is created, listen to the
:data:`~django.db.models.signals.m2m_changed` signal.
.. versionchanged:: 1.9
The ``bulk`` parameter was added. In older versions, foreign key
updates were always done using ``save()``. Use ``bulk=False`` if
you require the old behavior.
.. method:: create(**kwargs)
Creates a new object, saves it and puts it in the related object set.
Returns the newly created object::
>>> b = Blog.objects.get(id=1)
>>> e = b.entry_set.create(
... headline='Hello',
... body_text='Hi',
... pub_date=datetime.date(2005, 1, 1)
... )
# No need to call e.save() at this point -- it's already been saved.
This is equivalent to (but much simpler than)::
>>> b = Blog.objects.get(id=1)
>>> e = Entry(
... blog=b,
... headline='Hello',
... body_text='Hi',
... pub_date=datetime.date(2005, 1, 1)
... )
>>> e.save(force_insert=True)
Note that there's no need to specify the keyword argument of the model
that defines the relationship. In the above example, we don't pass the
parameter ``blog`` to ``create()``. Django figures out that the new
``Entry`` object's ``blog`` field should be set to ``b``.
.. method:: remove(*objs)
Removes the specified model objects from the related object set::
>>> b = Blog.objects.get(id=1)
>>> e = Entry.objects.get(id=234)
>>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.
Similar to :meth:`add()`, ``e.save()`` is called in the example above
to perform the update. Using ``remove()`` with a many-to-many
relationship, however, will delete the relationships using
:meth:`QuerySet.delete()<django.db.models.query.QuerySet.delete>` which
means no model ``save()`` methods are called; listen to the
:data:`~django.db.models.signals.m2m_changed` signal if you wish to
execute custom code when a relationship is deleted.
For :class:`~django.db.models.ForeignKey` objects, this method only
exists if ``null=True``. If the related field can't be set to ``None``
(``NULL``), then an object can't be removed from a relation without
being added to another. In the above example, removing ``e`` from
``b.entry_set()`` is equivalent to doing ``e.blog = None``, and because
the ``blog`` :class:`~django.db.models.ForeignKey` doesn't have
``null=True``, this is invalid.
.. versionchanged 1.7::
For :class:`~django.db.models.ForeignKey` objects, this method accepts
a ``bulk`` argument to control how to perform the operation.
If ``True`` (the default), ``QuerySet.update()`` is used.
If ``bulk=False``, the ``save()`` method of each individual model
instance is called instead. This triggers the
:data:`~django.db.models.signals.pre_save` and
:data:`~django.db.models.signals.post_save` signals and comes at the
expense of performance.
.. method:: clear()
Removes all objects from the related object set::
>>> b = Blog.objects.get(id=1)
>>> b.entry_set.clear()
Note this doesn't delete the related objects -- it just disassociates
them.
Just like ``remove()``, ``clear()`` is only available on
:class:`~django.db.models.ForeignKey`\s where ``null=True`` and it also
accepts the ``bulk`` keyword argument.
.. method:: set(objs, bulk=True, clear=False)
.. versionadded:: 1.9
Replace the set of related objects::
>>> new_list = [obj1, obj2, obj3]
>>> e.related_set.set(new_list)
This method accepts a ``clear`` argument to control how to perform the
operation. If ``False`` (the default), the elements missing from the
new set are removed using ``remove()`` and only the new ones are added.
If ``clear=True``, the ``clear()`` method is called instead and the
whole set is added at once.
The ``bulk`` argument is passed on to :meth:`add`.
Note that since ``set()`` is a compound operation, it is subject to
race conditions. For instance, new objects may be added to the database
in between the call to ``clear()`` and the call to ``add()``.
.. note::
Note that ``add()``, ``create()``, ``remove()``, ``clear()``, and
``set()`` all apply database changes immediately for all types of
related fields. In other words, there is no need to call ``save()``
on either end of the relationship.
Also, if you are using :ref:`an intermediate model
<intermediary-manytomany>` for a many-to-many relationship, some of the
related manager's methods are disabled.
Direct Assignment
-----------------
A related object set can be replaced in bulk with one operation by assigning a
new iterable of objects to it::
>>> new_list = [obj1, obj2, obj3]
>>> e.related_set = new_list
If the foreign key relationship has ``null=True``, then the related manager
will first disassociate any existing objects in the related set before adding
the contents of ``new_list``. Otherwise the objects in ``new_list`` will be
added to the existing related object set.
.. versionchanged:: 1.9
In earlier versions, direct assignment used to perform ``clear()`` followed
by ``add()``. It now performs a ``set()`` with the keyword argument
``clear=False``.
.. deprecated:: 1.10
Direct assignment is deprecated in favor of the
:meth:`~django.db.models.fields.related.RelatedManager.set` method::
>>> e.related_set.set([obj1, obj2, obj3])
This prevents confusion about an assignment resulting in an implicit save.
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