InstanceState default path is RootRegistry, not tuple

Fixed regression caused in 1.3.13 by :ticket:`5056` where a refactor of the
ORM path registry system made it such that a path could no longer be
compared to an empty tuple, which can occur in a particular kind of joined
eager loading path.   The "empty tuple" use case has been resolved so that
the path registry is compared to a path registry in all cases;  the
:class:`.PathRegistry` object itself now implements ``__eq__()`` and
``__ne__()`` methods which will take place for all equality comparisons and
continue to succeed in the not anticipated case that a non-
:class:`.PathRegistry` object is compared, while emitting a warning that
this object should not be the subject of the comparison.

Fixes: #5110
Change-Id: I6cab6cd771c131d12b17939b369212f12c6bee16

2 files changed, 19 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/path_registry.py b/lib/sqlalchemy/orm/path_registry.py
index 2c0847426..59ae082bb 100644
--- a/lib/sqlalchemy/orm/path_registry.py
+++ b/lib/sqlalchemy/orm/path_registry.py
@@ -65,7 +65,24 @@ class PathRegistry(HasCacheKey):
     ]
 
     def __eq__(self, other):
-        return other is not None and self.path == other.path
+        try:
+            return other is not None and self.path == other.path
+        except AttributeError:
+            util.warn(
+                "Comparison of PathRegistry to %r is not supported"
+                % (type(other))
+            )
+            return False
+
+    def __ne__(self, other):
+        try:
+            return other is None or self.path != other.path
+        except AttributeError:
+            util.warn(
+                "Comparison of PathRegistry to %r is not supported"
+                % (type(other))
+            )
+            return True
 
     def set(self, attributes, key, value):
         log.debug("set '%s' on path '%s' to '%s'", key, self, value)
diff --git a/lib/sqlalchemy/orm/state.py b/lib/sqlalchemy/orm/state.py
index 830b36770..40f8b197e 100644
--- a/lib/sqlalchemy/orm/state.py
+++ b/lib/sqlalchemy/orm/state.py
@@ -62,7 +62,7 @@ class InstanceState(interfaces.InspectionAttrInfo):
     key = None
     runid = None
     load_options = util.EMPTY_SET
-    load_path = ()
+    load_path = PathRegistry.root
     insert_order = None
     _strong_obj = None
     modified = False