Raise informative exception for non-sortable PK

An informative exception is re-raised when a primary key value is not
sortable in Python during an ORM flush under Python 3, such as an ``Enum``
that has no ``__lt__()`` method; normally Python 3 raises a ``TypeError``
in this case.   The flush process sorts persistent objects by primary key
in Python so the values must be sortable.

Change-Id: Ia186968982dcd1234b82f2e701fefa2a1668a7e4
Fixes: #4232

1 files changed, 7 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/persistence.py b/lib/sqlalchemy/orm/persistence.py
index 4f1e08afa..a48bf9bf7 100644
--- a/lib/sqlalchemy/orm/persistence.py
+++ b/lib/sqlalchemy/orm/persistence.py
@@ -1273,8 +1273,14 @@ def _sort_states(states):
     pending = set(states)
     persistent = set(s for s in pending if s.key is not None)
     pending.difference_update(persistent)
+    try:
+        persistent_sorted = sorted(persistent, key=lambda q: q.key[1])
+    except TypeError as err:
+        raise sa_exc.InvalidRequestError(
+            "Could not sort objects by primary key; primary key "
+            "values must be sortable in Python (was: %s)" % err)
     return sorted(pending, key=operator.attrgetter("insert_order")) + \
-        sorted(persistent, key=lambda q: q.key[1])
+        persistent_sorted
 
 
 class BulkUD(object):