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

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

1 files changed, 23 insertions, 1 deletions

diff --git a/docs/releases/1.9.txt b/docs/releases/1.9.txt
index eee63c6f6d..d00bfef096 100644
--- a/docs/releases/1.9.txt
+++ b/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
 ~~~~~~~~~~~~~