Removed a redundant example in contenttypes docs.

    >>> TaggedItem.objects.filter(bookmarks__url__contains='django')

    <QuerySet [<TaggedItem: django>, <TaggedItem: python>]>

Of course, if you don't add the reverse relationship, you can do the

same types of lookups manually::

    >>> b = Bookmark.objects.get(url='https://www.djangoproject.com/')

    >>> bookmark_type = ContentType.objects.get_for_model(b)

    >>> TaggedItem.objects.filter(content_type__pk=bookmark_type.id, object_id=b.id)

    <QuerySet [<TaggedItem: django>, <TaggedItem: python>]>

Just as :class:`~django.contrib.contenttypes.fields.GenericForeignKey`

accepts the names of the content-type and object-ID fields as

arguments, so too does

``object_primary_key`` to create its generic foreign key, then a

:class:`.GenericRelation` back to it would need to be defined like so::

    tags = GenericRelation(TaggedItem,

    tags = GenericRelation(

        TaggedItem,

        content_type_field='content_type_fk',

                           object_id_field='object_primary_key')

Of course, if you don't add the reverse relationship, you can do the

same types of lookups manually::

    >>> b = Bookmark.objects.get(url='https://www.djangoproject.com/')

    >>> bookmark_type = ContentType.objects.get_for_model(b)

    >>> TaggedItem.objects.filter(content_type__pk=bookmark_type.id,

    ...                           object_id=b.id)

    <QuerySet [<TaggedItem: django>, <TaggedItem: python>]>

Note that if the model in a

:class:`~django.contrib.contenttypes.fields.GenericRelation` uses a

non-default value for ``ct_field`` or ``fk_field`` in its

:class:`~django.contrib.contenttypes.fields.GenericForeignKey` (for example, if

you had a ``Comment`` model that uses ``ct_field="object_pk"``),

you'll need to set ``content_type_field`` and/or ``object_id_field`` in

the :class:`~django.contrib.contenttypes.fields.GenericRelation` to

match the ``ct_field`` and ``fk_field``, respectively, in the

:class:`~django.contrib.contenttypes.fields.GenericForeignKey`::

    comments = fields.GenericRelation(Comment, object_id_field="object_pk")

        object_id_field='object_primary_key',

    )

Note also, that if you delete an object that has a

:class:`~django.contrib.contenttypes.fields.GenericRelation`, any objects