Add tests for issue #8168; slight internal adjustments

The issue in #8168 was improved, but not completely fixed,
by #8456.

This includes some small changes to ORM context that
are a prerequisite for getting ORM adaptation to be
better.   Have these in 2.0.0b4 so that we have at
least a better starting point.

References: #8168
Change-Id: I51dbe333b156048836d074fbba1d850f9eb67fd2

3 files changed, 13 insertions, 11 deletions

diff --git a/lib/sqlalchemy/orm/context.py b/lib/sqlalchemy/orm/context.py
index 621b3e5d7..b5b326bca 100644
--- a/lib/sqlalchemy/orm/context.py
+++ b/lib/sqlalchemy/orm/context.py
@@ -520,13 +520,16 @@ class ORMCompileState(AbstractORMCompileState):
             for mp in ext_info.mapper.iterate_to_root():
                 self._mapper_loads_polymorphically_with(
                     mp,
-                    sql_util.ColumnAdapter(selectable, mp._equivalent_columns),
+                    ORMAdapter(
+                        mp, mp._equivalent_columns, selectable=selectable
+                    ),
                 )
 
     def _mapper_loads_polymorphically_with(self, mapper, adapter):
         for m2 in mapper._with_polymorphic_mappers or [mapper]:
             self._polymorphic_adapters[m2] = adapter
-            for m in m2.iterate_to_root():  # TODO: redundant ?
+
+            for m in m2.iterate_to_root():
                 self._polymorphic_adapters[m.local_table] = adapter
 
     @classmethod
@@ -1673,13 +1676,6 @@ class ORMSelectCompileState(ORMCompileState, SelectState):
 
                 left = onclause._parententity
 
-                alias = self._polymorphic_adapters.get(left, None)
-
-                # could be None or could be ColumnAdapter also
-                if isinstance(alias, ORMAdapter) and alias.mapper.isa(left):
-                    left = alias.aliased_class
-                    onclause = getattr(left, onclause.key)
-
                 prop = onclause.property
                 if not isinstance(onclause, attributes.QueryableAttribute):
                     onclause = prop
diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index c0fc11d23..6ed8e2272 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -107,6 +107,7 @@ if typing.TYPE_CHECKING:
     from ..sql.selectable import _ColumnsClauseElement
     from ..sql.selectable import Alias
     from ..sql.selectable import Select
+    from ..sql.selectable import Selectable
     from ..sql.selectable import Subquery
     from ..sql.visitors import anon_map
     from ..util.typing import _AnnotationScanType
@@ -456,10 +457,12 @@ class ORMAdapter(sql_util.ColumnAdapter):
         adapt_required: bool = False,
         allow_label_resolve: bool = True,
         anonymize_labels: bool = False,
+        selectable: Optional[Selectable] = None,
     ):
 
         self.mapper = entity.mapper
-        selectable = entity.selectable
+        if selectable is None:
+            selectable = entity.selectable
         if insp_is_aliased_class(entity):
             self.is_aliased_class = True
             self.aliased_insp = entity
@@ -478,6 +481,10 @@ class ORMAdapter(sql_util.ColumnAdapter):
         )
 
     def _include_fn(self, elem):
+        # TODO: we still have cases where we should return False here
+        # yet we are not able to reliably detect without false positives.
+        # see issue #8168
+
         entity = elem._annotations.get("parentmapper", None)
 
         return not entity or entity.isa(self.mapper) or self.mapper.isa(entity)
diff --git a/lib/sqlalchemy/sql/util.py b/lib/sqlalchemy/sql/util.py
index 14cbe2456..d162428ec 100644
--- a/lib/sqlalchemy/sql/util.py
+++ b/lib/sqlalchemy/sql/util.py
@@ -1139,7 +1139,6 @@ class ClauseAdapter(visitors.ReplacingExternalTraversal):
         if isinstance(col, FromClause) and not isinstance(
             col, functions.FunctionElement
         ):
-
             if self.selectable.is_derived_from(col):
                 if self.adapt_from_selectables:
                     for adp in self.adapt_from_selectables: