[1.10.x] Refs #25550 -- Corrected deprecation message for assigning M2M relations.

Backport of 5fa4370543 from master

django/db/models/fields/related_descriptors.py

@@ -475,6 +475,12 @@ class ReverseManyToOneDescriptor(object):
 
         return self.related_manager_cls(instance)
 
+    def _get_set_deprecation_msg_params(self):
+        return (  # RemovedInDjango20Warning
+            'reverse side of a related set',
+            self.rel.get_accessor_name(),
+        )
+
     def __set__(self, instance, value):
         """
         Set the related objects through the reverse relation.
@@ -486,9 +492,9 @@ class ReverseManyToOneDescriptor(object):
         - ``value`` is the ``children`` sequence on the right of the equal sign
         """
         warnings.warn(
-            'Direct assignment to the reverse side of a related set is '
+            'Direct assignment to the %s is deprecated due to the implicit '
-            'deprecated due to the implicit save() that happens. Use %s.set() '
+            'save() that happens. Use %s.set() instead.' % self._get_set_deprecation_msg_params(),
-            'instead.' % self.rel.get_accessor_name(), RemovedInDjango20Warning, stacklevel=2,
+            RemovedInDjango20Warning, stacklevel=2,
         )
         manager = self.__get__(instance)
         manager.set(value)
@@ -723,6 +729,12 @@ class ManyToManyDescriptor(ReverseManyToOneDescriptor):
             reverse=self.reverse,
         )
 
+    def _get_set_deprecation_msg_params(self):
+        return (  # RemovedInDjango20Warning
+            '%s side of a many-to-many set' % ('reverse' if self.reverse else 'forward'),
+            self.rel.get_accessor_name() if self.reverse else self.field.name,
+        )
+
 
 def create_forward_many_to_many_manager(superclass, rel, reverse):
     """

tests/many_to_many/tests.py

@@ -400,15 +400,24 @@ class ManyToManyTests(TestCase):
         self.a4.publications.set([], clear=True)
         self.assertQuerysetEqual(self.a4.publications.all(), [])
 
-    def test_assign_deprecation(self):
+    def test_assign_forward_deprecation(self):
         msg = (
-            "Direct assignment to the reverse side of a related set is "
+            "Direct assignment to the reverse side of a many-to-many set is "
             "deprecated due to the implicit save() that happens. Use "
             "article_set.set() instead."
         )
         with self.assertRaisesMessage(RemovedInDjango20Warning, msg):
             self.p2.article_set = [self.a4, self.a3]
 
+    def test_assign_reverse_deprecation(self):
+        msg = (
+            "Direct assignment to the forward side of a many-to-many "
+            "set is deprecated due to the implicit save() that happens. Use "
+            "publications.set() instead."
+        )
+        with self.assertRaisesMessage(RemovedInDjango20Warning, msg):
+            self.a1.publications = [self.p1, self.p2]
+
     @ignore_warnings(category=RemovedInDjango20Warning)
     def test_assign_deprecated(self):
         self.p2.article_set = [self.a4, self.a3]