Merge pull request #1560 from technivore/ticket_20970

      In this example, the methods below will be available both on

      ``topping.pizza_set`` and on ``pizza.toppings``.

    These related managers have some extra methods:

.. _related-manager-methods:

Related Manager Methods

-----------------------

.. method:: add(obj1, [obj2, ...])

        >>> e = Entry.objects.get(id=234)

        >>> b.entry_set.add(e) # Associates Entry e with Blog b.

        In the example above, ``e.save()`` is called to perform the update.

    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()

    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.