Cancel polymorphic loading in optimized get

Since optimized_get for inheriting mappers writes a simple
SELECT, we need to cancel out any with_polymorphic selectables
that interfere with simple column lookup.  While adaptation is
another option, just removing the with_polymorphic is much
simpler.   The issue is not noticeable unless the ResultProxy
is not allowing "key fallback" column lookups, which will
be the case when this behavior is deprecated.

Fixes: #4718
Change-Id: I8fa2f5c0434b6a681813a92ac71fe12712f5d634

4 files changed, 11 insertions, 4 deletions

diff --git a/lib/sqlalchemy/orm/loading.py b/lib/sqlalchemy/orm/loading.py
index 94a9b8d22..886240591 100644
--- a/lib/sqlalchemy/orm/loading.py
+++ b/lib/sqlalchemy/orm/loading.py
@@ -23,6 +23,7 @@ from . import strategy_options
 from .base import _DEFER_FOR_STATE
 from .base import _SET_DEFERRED_EXPIRED
 from .util import _none_set
+from .util import aliased
 from .util import state_str
 from .. import exc as sa_exc
 from .. import util
@@ -633,7 +634,6 @@ def _instance_processor(
     if mapper.polymorphic_map and not _polymorphic_from and not refresh_state:
         # if we are doing polymorphic, dispatch to a different _instance()
         # method specific to the subclass mapper
-
         def ensure_no_pk(row):
             identitykey = (
                 identity_class,
@@ -957,9 +957,10 @@ def load_scalar_attributes(mapper, state, attribute_names):
         # by default
         statement = mapper._optimized_get_statement(state, attribute_names)
         if statement is not None:
+            wp = aliased(mapper, statement)
             result = load_on_ident(
-                session.query(mapper)
-                .options(strategy_options.Load(mapper).undefer("*"))
+                session.query(wp)
+                .options(strategy_options.Load(wp).undefer("*"))
                 .from_statement(statement),
                 None,
                 only_load_props=attribute_names,
diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index 3d08dce22..0480bfb01 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -774,6 +774,11 @@ class Query(Generative):
             )
         entity = self._entities[0]._clone()
         self._entities = [entity] + self._entities[1:]
+
+        # NOTE: we likely should set primary_entity here, however
+        # this hasn't been changed for many years and we'd like to
+        # deprecate this method.
+
         entity.set_with_polymorphic(
             self,
             cls_or_mappers,
diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 1f2f65728..1c7c0546d 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -183,7 +183,6 @@ class ColumnLoader(LoaderStrategy):
         memoized_populators,
         **kwargs
     ):
-
         for c in self.columns:
             if adapter:
                 c = adapter.columns[c]
diff --git a/lib/sqlalchemy/sql/util.py b/lib/sqlalchemy/sql/util.py
index 3c7f904de..780cdc7b2 100644
--- a/lib/sqlalchemy/sql/util.py
+++ b/lib/sqlalchemy/sql/util.py
@@ -798,6 +798,8 @@ class ClauseAdapter(visitors.ReplacingCloningVisitor):
                 if newcol is not None:
                     return newcol
         if self.adapt_on_names and newcol is None:
+            # TODO: this should be changed to .exported_columns if and
+            # when we need to be able to adapt a plain Select statement
             newcol = self.selectable.c.get(col.name)
         return newcol