=========================
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)

      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(obj1, [obj2, ...])

        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,
        ``e.save()`` is called by the related manager to perform the update.
        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.

    .. 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(obj1, [obj2, ...])

        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.

    .. 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``.

    .. note::

       Note that ``add()``, ``create()``, ``remove()``, and ``clear()`` 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.

.. _direct-assignment:

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 call ``clear()`` to 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.
