Fix boolean check in new path comparison logic

Fixed regression where a new error message that was supposed to raise when
attempting to link a relationship option to an AliasedClass without using
:meth:`.PropComparator.of_type` would instead raise an ``AttributeError``.
Note that in 1.3, it is no longer valid to create an option path from a
plain mapper relationship to an :class:`.AliasedClass` without using
:meth:`.PropComparator.of_type`.

Fixes: #4566
Change-Id: Ic547a1c8408e41aec66ef9644aac7f76f50dd064

2 files changed, 22 insertions, 4 deletions

diff --git a/lib/sqlalchemy/orm/strategy_options.py b/lib/sqlalchemy/orm/strategy_options.py
index 8a34771c7..912b6b550 100644
--- a/lib/sqlalchemy/orm/strategy_options.py
+++ b/lib/sqlalchemy/orm/strategy_options.py
@@ -227,7 +227,7 @@ class Load(Generative, MapperOption):
                 if raiseerr:
                     raise sa_exc.ArgumentError(
                         'Can\'t find property named "%s" on '
-                        "%s in this Query. " % (attr, ent)
+                        "%s in this Query." % (attr, ent)
                     )
                 else:
                     return None
@@ -236,6 +236,9 @@ class Load(Generative, MapperOption):
 
             path = path[attr]
         elif _is_mapped_class(attr):
+            # TODO: this does not appear to be a valid codepath.  "attr"
+            # would never be a mapper.  This block is present in 1.2
+            # as well howver does not seem to be accessed in any tests.
             if not orm_util._entity_corresponds_to_use_path_impl(
                 attr.parent, path[-1]
             ):
@@ -255,7 +258,20 @@ class Load(Generative, MapperOption):
                 if raiseerr:
                     raise sa_exc.ArgumentError(
                         'Attribute "%s" does not '
-                        'link from element "%s"' % (attr, path.entity)
+                        'link from element "%s".%s'
+                        % (
+                            attr,
+                            path.entity,
+                            (
+                                "  Did you mean to use "
+                                "%s.of_type(%s)?"
+                                % (path[-2], attr.class_.__name__)
+                                if len(path) > 1
+                                and path.entity.is_mapper
+                                and attr.parent.is_aliased_class
+                                else ""
+                            ),
+                        )
                     )
                 else:
                     return None
diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index f9258895d..7b8edd349 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -1265,8 +1265,10 @@ def _entity_corresponds_to_use_path_impl(given, entity):
         return (
             entity.is_aliased_class
             and not entity._use_mapper_path
-            and given is entity
-            or given in entity._with_polymorphic_entities
+            and (
+                given is entity
+                or given in entity._with_polymorphic_entities
+            )
         )
     elif not entity.is_aliased_class:
         return given.common_parent(entity.mapper)