warn on duplicate polymorphic_identity

2 files changed, 22 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index 863dab5cb..0e0c1a833 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -974,6 +974,15 @@ class Mapper(InspectionAttr):
             self._all_tables = self.inherits._all_tables
 
             if self.polymorphic_identity is not None:
+                if self.polymorphic_identity in self.polymorphic_map:
+                    util.warn(
+                        "Reassigning polymorphic association for identity %r "
+                        "from %r to %r: Check for duplicate use of %r as "
+                        "value for polymorphic_identity." %
+                        (self.polymorphic_identity,
+                         self.polymorphic_map[self.polymorphic_identity],
+                         self, self.polymorphic_identity)
+                )
                 self.polymorphic_map[self.polymorphic_identity] = self
 
         else:
diff --git a/test/orm/test_mapper.py b/test/orm/test_mapper.py
index 63ba1a207..264b386d4 100644
--- a/test/orm/test_mapper.py
+++ b/test/orm/test_mapper.py
@@ -716,6 +716,19 @@ class MapperTest(_fixtures.FixtureTest, AssertsCompiledSQL):
             m3.identity_key_from_instance(AddressUser())
         )
 
+    def test_reassign_polymorphic_identity_warns(self):
+        User = self.classes.User
+        users = self.tables.users
+        class MyUser(User):
+            pass
+        m1 = mapper(User, users, polymorphic_on=users.c.name,
+                    polymorphic_identity='user')
+        assert_raises_message(
+            sa.exc.SAWarning,
+            "Reassigning polymorphic association for identity 'user'",
+            mapper,
+            MyUser, users, inherits=User, polymorphic_identity='user'
+        )
 
 
     def test_illegal_non_primary(self):