Fixed #24208 -- Documented changes in private model relations.

``django.template.loader.find_template()``, and

``django.template.loader.get_template_from_string()`` were removed.

``model`` attribute on private model relations

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In earlier versions of Django, on a model with a reverse foreign key

relationship (for example), ``model._meta.get_all_related_objects()`` returned

the relationship as a ``django.db.models.related.RelatedObject`` with the

``model`` attribute set to the source of the relationship. Now, this method

returns the relationship as ``django.db.models.fields.related.ManyToOneRel``

(private API ``RelatedObject`` has been removed), and the ``model`` attribute

is set to the target of the relationship instead of the source. The source

model is accessible on the ``related_model`` attribute instead.

Consider this example from the tutorial in Django 1.8::

    >>> p = Poll.objects.get(pk=1)

    >>> p._meta.get_all_related_objects()

    [<ManyToOneRel: polls.choice>]

    >>> p._meta.get_all_related_objects()[0].model

    <class 'polls.models.Poll'>

    >>> p._meta.get_all_related_objects()[0].related_model

    <class 'polls.models.Choice'>

and compare it to the behavior on older versions::

    >>> p._meta.get_all_related_objects()

    [<RelatedObject: polls:choice related to poll>]

    >>> p._meta.get_all_related_objects()[0].model

    <class 'polls.models.Choice'>

To access the source model, you can use a pattern like this to write code that

will work with both Django 1.8 and older versions::

    for relation in opts.get_all_related_objects():

        to_model = getattr(relation, 'related_model', relation.model)

Also note that ``get_all_related_objects()`` is deprecated in 1.8. See the

:ref:`upgrade guide <migrating-old-meta-api>` for the new API.

Database backend API

~~~~~~~~~~~~~~~~~~~~