Raise at Core / ORM concrete inh level for label overlap

Fixed regression where the :class:`.ConcreteBase` would fail to map at all
when a mapped column name overlapped with the discriminator column name,
producing an assertion error. The use case here did not function correctly
in 1.3 as the polymorphic union would produce a query that ignored the
discriminator column entirely, while emitting duplicate column warnings. As
1.4's architecture cannot easily reproduce this essentially broken behavior
of 1.3 at the ``select()`` level right now, the use case now raises an
informative error message instructing the user to use the
``.ConcreteBase._concrete_discriminator_name`` attribute to resolve the
conflict. To assist with this configuration,
``.ConcreteBase._concrete_discriminator_name`` may be placed on the base
class only where it will be automatically used by subclasses; previously
this was not the case.

Fixes: #6090
Change-Id: I8b7d01e4c9ea0dc97f30b8cd658b3505b24312a7

1 files changed, 9 insertions, 5 deletions

diff --git a/lib/sqlalchemy/ext/declarative/extensions.py b/lib/sqlalchemy/ext/declarative/extensions.py
index 5c6356d8b..fd8bed6be 100644
--- a/lib/sqlalchemy/ext/declarative/extensions.py
+++ b/lib/sqlalchemy/ext/declarative/extensions.py
@@ -15,7 +15,6 @@ from ...orm import relationships
 from ...orm.base import _mapper_or_none
 from ...orm.clsregistry import _resolver
 from ...orm.decl_base import _DeferredMapperConfig
-from ...orm.decl_base import _get_immediate_cls_attr
 from ...orm.util import polymorphic_union
 from ...schema import Table
 from ...util import OrderedDict
@@ -86,6 +85,13 @@ class ConcreteBase(object):
        attribute to :class:`_declarative.ConcreteBase` so that the
        virtual discriminator column name can be customized.
 
+    .. versionchanged:: 1.4.2 The ``_concrete_discriminator_name`` attribute
+       need only be placed on the basemost class to take correct effect for
+       all subclasses.   An explicit error message is now raised if the
+       mapped column names conflict with the discriminator name, whereas
+       in the 1.3.x series there would be some warnings and then a non-useful
+       query would be generated.
+
     .. seealso::
 
         :class:`.AbstractConcreteBase`
@@ -112,8 +118,7 @@ class ConcreteBase(object):
             return
 
         discriminator_name = (
-            _get_immediate_cls_attr(cls, "_concrete_discriminator_name")
-            or "type"
+            getattr(cls, "_concrete_discriminator_name", None) or "type"
         )
 
         mappers = list(m.self_and_descendants)
@@ -264,8 +269,7 @@ class AbstractConcreteBase(ConcreteBase):
                 mappers.append(mn)
 
         discriminator_name = (
-            _get_immediate_cls_attr(cls, "_concrete_discriminator_name")
-            or "type"
+            getattr(cls, "_concrete_discriminator_name", None) or "type"
         )
         pjoin = cls._create_polymorphic_union(mappers, discriminator_name)