Convert expression type for concat + Enum

Fixed bug involving new value translation and validation feature
in :class:`.Enum` whereby using the enum object in a string
concatenation would maintain the :class:`.Enum` type as the type
of the expression overall, producing missing lookups.  A string
concatenation against an :class:`.Enum`-typed column now uses
:class:`.String` as the datatype of the expression itself.

Change-Id: Id402054e3ef008e0250c740dbb7e1c80f339fe78
Fixes: #3833

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/sqltypes.py b/lib/sqlalchemy/sql/sqltypes.py
index cae23902b..ef1624fa0 100644
--- a/lib/sqlalchemy/sql/sqltypes.py
+++ b/lib/sqlalchemy/sql/sqltypes.py
@@ -1296,6 +1296,19 @@ class Enum(String, SchemaType):
                 raise LookupError(
                     '"%s" is not among the defined enum values' % elem)
 
+    class Comparator(String.Comparator):
+
+        def _adapt_expression(self, op, other_comparator):
+            op, typ = super(Enum.Comparator, self)._adapt_expression(
+                op, other_comparator)
+            if op is operators.concat_op:
+                typ = String(
+                    self.type.length,
+                    convert_unicode=self.type.convert_unicode)
+            return op, typ
+
+    comparator_factory = Comparator
+
     def _object_value_for_elem(self, elem):
         try:
             return self._object_lookup[elem]