Fix bug when discriminator column is used as a part of a primary key:

http://stackoverflow.com/questions/42860579/return-none-as-classtype-in-entity-inheritance-on-ponyorm

pony/orm/core.py

@@ -3881,19 +3881,24 @@ def _set_rbits(entity, objects, attrs):
             obj._rbits_ |= rbits & ~wbits
     def _parse_row_(entity, row, attr_offsets):
         discr_attr = entity._discriminator_attr_
-        if not discr_attr: real_entity_subclass = entity
+        if not discr_attr:
+            discr_value = None
+            real_entity_subclass = entity
         else:
             discr_offset = attr_offsets[discr_attr][0]
             discr_value = discr_attr.validate(row[discr_offset], None, entity, from_db=True)
             real_entity_subclass = discr_attr.code2cls[discr_value]
+            discr_value = real_entity_subclass._discriminator_  # To convert unicode to str in Python 2.x
 
         avdict = {}
         for attr in real_entity_subclass._attrs_:
             offsets = attr_offsets.get(attr)
             if offsets is None or attr.is_discriminator: continue
             avdict[attr] = attr.parse_value(row, offsets)
-        if not entity._pk_is_composite_: pkval = avdict.pop(entity._pk_attrs_[0], None)
-        else: pkval = tuple(avdict.pop(attr, None) for attr in entity._pk_attrs_)
+
+        pkval = tuple(avdict.pop(attr, discr_value) for attr in entity._pk_attrs_)
+        assert None not in pkval
+        if not entity._pk_is_composite_: pkval = pkval[0]
         return real_entity_subclass, pkval, avdict
     def _load_many_(entity, objects):
         database = entity._database_

pony/orm/tests/test_inheritance.py

@@ -257,5 +257,29 @@ class Entity3(Entity1):
             result = select(e for e in Entity1 if e.b == 30 or e.c == 50)
             self.assertEqual([ e.id for e in result ], [ 2, 3 ])
 
+    def test_discriminator_1(self):
+        db = Database('sqlite', ':memory:')
+        class Entity1(db.Entity):
+            a = Discriminator(str)
+            b = Required(int)
+            PrimaryKey(a, b)
+        class Entity2(db.Entity1):
+            c = Required(int)
+        db.generate_mapping(create_tables=True)
+        with db_session:
+            x = Entity1(b=10)
+            y = Entity2(b=20, c=30)
+        with db_session:
+            obj = Entity1.get(b=20)
+            self.assertEqual(obj.a, 'Entity2')
+            self.assertEqual(obj.b, 20)
+            self.assertEqual(obj._pkval_, ('Entity2', 20))
+        with db_session:
+            obj = Entity1['Entity2', 20]
+            self.assertEqual(obj.a, 'Entity2')
+            self.assertEqual(obj.b, 20)
+            self.assertEqual(obj._pkval_, ('Entity2', 20))
+
+
 if __name__ == '__main__':
     unittest.main()