- The :meth:`.PropComparator.of_type` modifier has been

improved in conjunction with loader directives such as
:func:`.joinedload` and :func:`.contains_eager` such that if
two :meth:`.PropComparator.of_type` modifiers of the same
base type/path are encountered, they will be joined together
into a single "polymorphic" entity, rather than replacing
the entity of type A with the one of type B.  E.g.
a joinedload of ``A.b.of_type(BSub1)->BSub1.c`` combined with
joinedload of ``A.b.of_type(BSub2)->BSub2.c`` will create a
single joinedload of ``A.b.of_type((BSub1, BSub2)) -> BSub1.c, BSub2.c``,
without the need for the ``with_polymorphic`` to be explicit
in the query.
fixes #3256

3 files changed, 29 insertions, 7 deletions

diff --git a/lib/sqlalchemy/orm/strategy_options.py b/lib/sqlalchemy/orm/strategy_options.py
index a4107202e..276da2ae0 100644
--- a/lib/sqlalchemy/orm/strategy_options.py
+++ b/lib/sqlalchemy/orm/strategy_options.py
@@ -161,11 +161,14 @@ class Load(Generative, MapperOption):
                 ext_info = inspect(ac)
 
                 path_element = ext_info.mapper
+                existing = path.entity_path[prop].get(
+                    self.context, "path_with_polymorphic")
                 if not ext_info.is_aliased_class:
                     ac = orm_util.with_polymorphic(
                         ext_info.mapper.base_mapper,
                         ext_info.mapper, aliased=True,
-                        _use_mapper_path=True)
+                        _use_mapper_path=True,
+                        _existing_alias=existing)
                 path.entity_path[prop].set(
                     self.context, "path_with_polymorphic", inspect(ac))
                 path = path[prop][path_element]
diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index ad610a4ac..4be8d19ff 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -543,8 +543,13 @@ class AliasedInsp(InspectionAttr):
                 mapper, self)
 
     def __repr__(self):
-        return '<AliasedInsp at 0x%x; %s>' % (
-            id(self), self.class_.__name__)
+        if self.with_polymorphic_mappers:
+            with_poly = "(%s)" % ", ".join(
+                mp.class_.__name__ for mp in self.with_polymorphic_mappers)
+        else:
+            with_poly = ""
+        return '<AliasedInsp at 0x%x; %s%s>' % (
+            id(self), self.class_.__name__, with_poly)
 
 
 inspection._inspects(AliasedClass)(lambda target: target._aliased_insp)
@@ -648,7 +653,8 @@ def aliased(element, alias=None, name=None, flat=False, adapt_on_names=False):
 def with_polymorphic(base, classes, selectable=False,
                      flat=False,
                      polymorphic_on=None, aliased=False,
-                     innerjoin=False, _use_mapper_path=False):
+                     innerjoin=False, _use_mapper_path=False,
+                     _existing_alias=None):
     """Produce an :class:`.AliasedClass` construct which specifies
     columns for descendant mappers of the given base.
 
@@ -713,6 +719,16 @@ def with_polymorphic(base, classes, selectable=False,
        only be specified if querying for one specific subtype only
     """
     primary_mapper = _class_to_mapper(base)
+    if _existing_alias:
+        assert _existing_alias.mapper is primary_mapper
+        classes = util.to_set(classes)
+        new_classes = set([
+            mp.class_ for mp in
+            _existing_alias.with_polymorphic_mappers])
+        if classes == new_classes:
+            return _existing_alias
+        else:
+            classes = classes.union(new_classes)
     mappers, selectable = primary_mapper.\
         _with_polymorphic_args(classes, selectable,
                                innerjoin=innerjoin)
diff --git a/lib/sqlalchemy/util/_collections.py b/lib/sqlalchemy/util/_collections.py
index a1fbc0fa0..d36852698 100644
--- a/lib/sqlalchemy/util/_collections.py
+++ b/lib/sqlalchemy/util/_collections.py
@@ -10,9 +10,10 @@
 from __future__ import absolute_import
 import weakref
 import operator
-from .compat import threading, itertools_filterfalse
+from .compat import threading, itertools_filterfalse, string_types
 from . import py2k
 import types
+import collections
 
 EMPTY_SET = frozenset()
 
@@ -779,10 +780,12 @@ def coerce_generator_arg(arg):
 def to_list(x, default=None):
     if x is None:
         return default
-    if not isinstance(x, (list, tuple)):
+    if not isinstance(x, collections.Iterable) or isinstance(x, string_types):
         return [x]
-    else:
+    elif isinstance(x, list):
         return x
+    else:
+        return list(x)
 
 
 def to_set(x):