Fixed #21169 -- Reworked RelatedManager methods use default filtering

The `remove()` and `clear()` methods of the related managers created by
`ForeignKey`, `GenericForeignKey`, and `ManyToManyField` suffered from a
number of issues. Some operations ran multiple data modifying queries without
wrapping them in a transaction, and some operations didn't respect default
filtering when it was present (i.e. when the default manager on the related
model implemented a custom `get_queryset()`).

Fixing the issues introduced some backward incompatible changes:

- The implementation of `remove()` for `ForeignKey` related managers changed
  from a series of `Model.save()` calls to a single `QuerySet.update()` call.
  The change means that `pre_save` and `post_save` signals aren't called anymore.

- The `remove()` and `clear()` methods for `GenericForeignKey` related
  managers now perform bulk delete so `Model.delete()` isn't called anymore.

- The `remove()` and `clear()` methods for `ManyToManyField` related
  managers perform nested queries when filtering is involved, which may
  or may not be an issue depending on the database and the data itself.

Refs. #3871, #21174.

Thanks Anssi Kääriäinen and Tim Graham for the reviews.

docs/ref/models/querysets.txt

@@ -2132,6 +2132,8 @@ extract two field values, where only one is expected::
     inner_qs = Blog.objects.filter(name__contains='Ch').values('name', 'id')
     entries = Entry.objects.filter(blog__name__in=inner_qs)
 
+.. _nested-queries-performance:
+
 .. admonition:: Performance considerations
 
     Be cautious about using nested queries and understand your database

docs/releases/1.7.txt

@@ -574,6 +574,32 @@ a :exc:`~exceptions.ValueError` when encountering them, you will have to
 install pytz_. You may be affected by this problem if you use Django's time
 zone-related date formats or :mod:`django.contrib.syndication`.
 
+``remove()`` and ``clear()`` methods of related managers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``remove()`` and ``clear()`` methods of the related managers created by
+``ForeignKey``, ``GenericForeignKey``, and ``ManyToManyField`` suffered from a
+number of issues. Some operations ran multiple data modifying queries without
+wrapping them in a transaction, and some operations didn't respect default
+filtering when it was present (i.e. when the default manager on the related
+model implemented a custom ``get_queryset()``).
+
+Fixing the issues introduced some backward incompatible changes:
+
+- The default implementation of ``remove()`` for ``ForeignKey`` related managers
+  changed from a series of ``Model.save()`` calls to a single
+  ``QuerySet.update()`` call. The change means that ``pre_save`` and
+  ``post_save`` signals aren't sent anymore.
+
+- The ``remove()`` and ``clear()`` methods for ``GenericForeignKey`` related
+  managers now perform bulk delete. The ``Model.delete()`` method isn't called
+  on each instance anymore.
+
+- The ``remove()`` and ``clear()`` methods for ``ManyToManyField`` related
+  managers perform nested queries when filtering is involved, which may or
+  may not be an issue depending on your database and your data itself.
+  See :ref:`this note <nested-queries-performance>` for more details.
+
 .. _pytz: https://pypi.python.org/pypi/pytz/
 
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