- Fixed bug where "middle" class in a polymorphic hierarchy

would have no 'polymorphic_on' column if it didn't also
specify a 'polymorphic_identity', leading to strange
errors upon refresh, wrong class loaded when querying
from that target. [ticket:2038]

4 files changed, 63 insertions, 12 deletions

diff --git a/CHANGES b/CHANGES
index 7ab5fbd02..7eca04fa8 100644
--- a/CHANGES
+++ b/CHANGES
@@ -191,6 +191,12 @@ CHANGES
     has no primary keys on the locally mapped table
     (but has pks on the superclass table).  [ticket:2019]
 
+  - Fixed bug where "middle" class in a polymorphic hierarchy
+    would have no 'polymorphic_on' column if it didn't also
+    specify a 'polymorphic_identity', leading to strange
+    errors upon refresh, wrong class loaded when querying
+    from that target. [ticket:2038]
+
 - sql
   - Column.copy(), as used in table.tometadata(), copies the 
     'doc' attribute.  [ticket:2028]
diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index c7e93c19d..63d2c4de9 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -305,16 +305,17 @@ class Mapper(object):
 
             if self.polymorphic_identity is not None:
                 self.polymorphic_map[self.polymorphic_identity] = self
-                if self.polymorphic_on is None:
-                    for mapper in self.iterate_to_root():
-                        # try to set up polymorphic on using
-                        # correesponding_column(); else leave
-                        # as None
-                        if mapper.polymorphic_on is not None:
-                            self.polymorphic_on = \
-                                    self.mapped_table.corresponding_column(
-                                                        mapper.polymorphic_on)
-                            break
+
+            if self.polymorphic_on is None:
+                for mapper in self.iterate_to_root():
+                    # try to set up polymorphic on using
+                    # correesponding_column(); else leave
+                    # as None
+                    if mapper.polymorphic_on is not None:
+                        self.polymorphic_on = \
+                                self.mapped_table.corresponding_column(
+                                                    mapper.polymorphic_on)
+                        break
         else:
             self._all_tables = set()
             self.base_mapper = self
diff --git a/test/ext/test_declarative.py b/test/ext/test_declarative.py
index d236cc3ab..cc743843f 100644
--- a/test/ext/test_declarative.py
+++ b/test/ext/test_declarative.py
@@ -1187,7 +1187,8 @@ class DeclarativeInheritanceTest(DeclarativeTestBase):
             primary_language = Column(String(50))
 
         assert 'inherits' not in Person.__mapper_args__
-        assert class_mapper(Engineer).polymorphic_on is None
+        assert class_mapper(Engineer).polymorphic_identity is None
+        assert class_mapper(Engineer).polymorphic_on is Person.__table__.c.type
 
     def test_custom_join_condition(self):
 
diff --git a/test/orm/inheritance/test_basic.py b/test/orm/inheritance/test_basic.py
index 7257cbd79..55cc43993 100644
--- a/test/orm/inheritance/test_basic.py
+++ b/test/orm/inheritance/test_basic.py
@@ -1516,6 +1516,50 @@ class PKDiscriminatorTest(_base.MappedTest):
         assert a.name=='a1new'
         assert p.name=='p1new'
 
+class NoPolyIdentInMiddleTest(_base.MappedTest):
+    @classmethod
+    def define_tables(cls, metadata):
+        Table('base', metadata,
+            Column('id', Integer, primary_key=True, test_needs_autoincrement=True),
+            Column('type', String(50), nullable=False),
+        )
+
+    @classmethod
+    def setup_classes(cls):
+        class A(_base.BasicEntity):
+            pass
+        class B(A):
+            pass
+        class C(B):
+            pass
+
+    @classmethod
+    @testing.resolve_artifact_names
+    def setup_mappers(cls):
+        mapper(A, base, polymorphic_on=base.c.type)
+        mapper(B, inherits=A)
+        mapper(C, inherits=B, polymorphic_identity='c')
+
+    @testing.resolve_artifact_names
+    def test_load_from_middle(self):
+        s = Session()
+        s.add(C())
+        o = s.query(B).first()
+        eq_(o.type, 'c')
+        assert isinstance(o, C)
+
+    @testing.resolve_artifact_names
+    def test_load_from_base(self):
+        s = Session()
+        s.add(C())
+        o = s.query(A).first()
+        eq_(o.type, 'c')
+        assert isinstance(o, C)
+
+    @testing.resolve_artifact_names
+    def test_discriminator(self):
+        assert class_mapper(B).polymorphic_on is base.c.type
+        assert class_mapper(C).polymorphic_on is base.c.type
 
 class DeleteOrphanTest(_base.MappedTest):
     """Test the fairly obvious, that an error is raised
@@ -1526,7 +1570,6 @@ class DeleteOrphanTest(_base.MappedTest):
 
     """
 
-
     @classmethod
     def define_tables(cls, metadata):
         global single, parent