Fixed problem with assiging an empty list to m2m related descriptors, introduced in [4231].

git-svn-id: http://code.djangoproject.com/svn/django/trunk@4232 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/fields/related.py

@@ -317,6 +317,8 @@ def create_many_related_manager(superclass):
             # *objs - objects to add
             from django.db import connection
 
+            # If there aren't any objects, there is nothing to do.
+            if objs:
                 # Add the newly created or already existing objects to the join table.
                 # First find out which items are already added, to avoid adding them twice
                 new_ids = set([obj._get_pk_val() for obj in objs])

tests/modeltests/many_to_many/models.py

@@ -231,4 +231,16 @@ __test__ = {'API_TESTS':"""
 >>> p1.article_set.all()
 [<Article: NASA uses Python>]
 
+# An alternate to calling clear() is to assign the empty set
+>>> p1.article_set = []
+>>> p1.article_set.all()
+[]
+
+>>> a2.publications = [p1, new_publication]
+>>> a2.publications.all()
+[<Publication: Highlights for Children>, <Publication: The Python Journal>]
+>>> a2.publications = []
+>>> a2.publications.all()
+[]
+
 """}