=========================

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 Blog(models.Model):

                # ...

                pass

            class Entry(models.Model):

                blog = models.ForeignKey(Blog, on_delete=models.CASCADE, null=True)

      In the above example, the methods below will be available on

      the manager ``blog.entry_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, through_defaults=None)

        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 (the ``bulk`` argument doesn't exist), 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, which will

        trigger ``pre_add`` and ``post_add`` actions.

        Using ``add()`` on a relation that already exists won't duplicate the

        relation, but it will still trigger signals.

        For many-to-many relationships ``add()`` accepts either model instances

        or field values, normally primary keys, as the ``*objs`` argument.

        Use the ``through_defaults`` argument to specify values for the new

        :ref:`intermediate model <intermediary-manytomany>` instance(s), if

        needed. You can use callables as values in the ``through_defaults``

        dictionary and they will be evaluated once before creating any

        intermediate instance(s).

    .. method:: create(through_defaults=None, **kwargs)

    .. method:: acreate(through_defaults=None, **kwargs)

        *Asynchronous version*: ``acreate``

        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 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``.

        Use the ``through_defaults`` argument to specify values for the new

        :ref:`intermediate model <intermediary-manytomany>` instance, if

        needed. You can use callables as values in the ``through_defaults``

        dictionary.

        .. versionchanged:: 4.1

            ``acreate()`` method was added.

    .. method:: remove(*objs, bulk=True)

        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 many-to-many relationships ``remove()`` accepts either model

        instances or field values, normally primary keys, as the ``*objs``

        argument.

        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.

        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.

        For many-to-many relationships, the ``bulk`` keyword argument doesn't

        exist.

    .. method:: clear(bulk=True)

        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.

        For many-to-many relationships, the ``bulk`` keyword argument doesn't

        exist.

    .. method:: set(objs, bulk=True, clear=False, through_defaults=None)

        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.

        For :class:`~django.db.models.ForeignKey` objects, the ``bulk``

        argument is passed on to :meth:`add` and :meth:`remove`.

        For many-to-many relationships, the ``bulk`` keyword argument doesn't

        exist.

        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()``.

        For many-to-many relationships ``set()`` accepts a list of either model

        instances or field values, normally primary keys, as the ``objs``

        argument.

        Use the ``through_defaults`` argument to specify values for the new

        :ref:`intermediate model <intermediary-manytomany>` instance(s), if

        needed. You can use callables as values in the ``through_defaults``

        dictionary and they will be evaluated once before creating any

        intermediate instance(s).

    .. 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.

       If you use :meth:`~django.db.models.query.QuerySet.prefetch_related`,

       the ``add()``, ``remove()``, ``clear()``, and ``set()`` methods clear

       the prefetched cache.