Fixed #6707 -- Added RelatedManager.set() and made descriptors' __set__ use it.

Thanks Anssi Kääriäinen, Carl Meyer, Collin Anderson, and Tim Graham for the reviews.

docs/ref/models/relations.txt

@@ -135,12 +135,31 @@ Related objects reference
         :class:`~django.db.models.ForeignKey`\s where ``null=True`` and it also
         accepts the ``bulk`` keyword argument.
 
+    .. method:: set(objs, clear=False)
+
+        .. versionadded:: 1.9
+
+        Replace the set of related objects::
+
+            >>> new_list = [obj1, obj2, obj3]
+            >>> e.related_set.set(new_list)
+
+        This method accepts a ``clear`` argument to control how to perform the
+        operation. If ``False`` (the default), the elements missing from the
+        new set are removed using ``remove()`` and only the new ones are added.
+        If ``clear=True``, the ``clear()`` method is called instead and the
+        whole set is added at once.
+
+        Note that since ``set()`` is a compound operation, it is subject to
+        race conditions. For instance, new objects may be added to the database
+        in between the call to ``clear()`` and the call to ``add()``.
+
     .. 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.
+       Note that ``add()``, ``create()``, ``remove()``, ``clear()``, and
+       ``set()`` 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.
 
        Also, if you are using :ref:`an intermediate model
        <intermediary-manytomany>` for a many-to-many relationship, some of the
@@ -158,6 +177,12 @@ new iterable of objects to it::
     >>> 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.
+will first 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.
+
+.. versionchanged:1.9
+
+    In earlier versions, direct assignment used to perform ``clear()`` followed
+    by ``add()``. It now performs a ``set()`` with the keyword argument
+    ``clear=False``.

docs/releases/1.9.txt

@@ -127,7 +127,10 @@ Management Commands
 Models
 ^^^^^^
 
-* ...
+* Added the :meth:`RelatedManager.set()
+  <django.db.models.fields.related.RelatedManager.set()>` method to the related
+  managers created by ``ForeignKey``, ``GenericForeignKey``, and
+  ``ManyToManyField``.
 
 Signals
 ^^^^^^^
@@ -192,6 +195,25 @@ used by the egg loader to detect if setuptools was installed. The ``is_usable``
 attribute is now removed and the egg loader instead fails at runtime if
 setuptools is not installed.
 
+Related set direct assignment
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+:ref:`Direct assignment <direct-assignment>`) used to perform a ``clear()``
+followed by a call to ``add()``. This caused needlessly large data changes
+and prevented using the :data:`~django.db.models.signals.m2m_changed` signal
+to track individual changes in many-to-many relations.
+
+Direct assignment now relies on the the new
+:meth:`django.db.models.fields.related.RelatedManager.set()` method on
+related managers which by default only processes changes between the
+existing related set and the one that's newly assigned. The previous behavior
+can be restored by replacing direct assignment by a call to ``set()`` with
+the keyword argument ``clear=True``.
+
+``ModelForm``, and therefore ``ModelAdmin``, internally rely on direct
+assignment for many-to-many relations and as a consequence now use the new
+behavior.
+
 Miscellaneous
 ~~~~~~~~~~~~~
 

docs/topics/db/queries.txt

@@ -1190,6 +1190,9 @@ be found in the :doc:`related objects reference </ref/models/relations>`.
 ``clear()``
     Removes all objects from the related object set.
 
+``set(objs)``
+    Replace the set of related objects.
+
 To assign the members of a related set in one fell swoop, just assign to it
 from any iterable object. The iterable can contain object instances, or just
 a list of primary key values. For example::