Ticket #14394: validate_bad_data_assignment_to_m2m.patch

django/db/models/fields/related.py

@@ -727 +727 @@
         if not self.field.rel.through._meta.auto_created:
             opts = self.field.rel.through._meta
             raise AttributeError("Cannot set values on a ManyToManyField which specifies an intermediary model.  Use %s.%s's Manager instead." % (opts.app_label, opts.object_name))
+        
+        if not isinstance(value, (list, tuple)):
+            raise TypeError(u"Cannot set %s values on a ManyToManyField. Only lists or tuples (or subclasses) allowed." % type(value))
 
         manager = self.__get__(instance)
         manager.clear()

tests/regressiontests/m2m_regress/tests.py

@@ -73 +73 @@
 
         self.assertQuerysetEqual(c1.tags.all(), ["<Tag: t1>", "<Tag: t2>"])
         self.assertQuerysetEqual(t1.tag_collections.all(), ["<TagCollection: c1>"])
+
+    def test_assigning_invalid_data_to_m2m_doesnt_clear_existing_relations(self):
+        
+        t1 = Tag.objects.create(name='t1') 
+        t2 = Tag.objects.create(name='t2') 
+        
+        c1 = TagCollection.objects.create(name='c1')
+        c1.tags = [t1,t2]
+        
+        def invalid_assigment():
+            c1.tags = 7
+
+        self.assertRaises(TypeError, invalid_assigment)
+
+        c1 = TagCollection.objects.get(pk=c1.pk)
+
+        self.assertQuerysetEqual(c1.tags.all(), ["<Tag: t1>", "<Tag: t2>"])
+
+