Fixed a breakage with ManyToManyFields in admin caused by r10139.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10170 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/fields/related.py

@@ -644,6 +644,14 @@ class ManyToManyRel(object):
         self.multiple = True
         self.through = through
 
+    def get_related_field(self):
+        """
+        Returns the field in the to' object to which this relationship is tied
+        (this is always the primary key on the target model). Provided for
+        symmetry with ManyToOneRel.
+        """
+        return self.to._meta.pk
+
 class ForeignKey(RelatedField, Field):
     empty_strings_allowed = False
     def __init__(self, to, to_field=None, rel_class=ManyToOneRel, **kwargs):