Only use schema_translate_map on SchemaItem subclasses

Fixed bug in new "schema translate" feature where the translated schema
name would be invoked in terms of an alias name when rendered along
with a column expression; occurred only when the source translate
name was "None".   The "schema translate" feature now only takes
effect for :class:`.SchemaItem` and :class:`.SchemaType` subclasses,
that is, objects that correspond to a DDL-creatable structure in
a database.

Change-Id: Ie8cb35aeaba2c67efec8c8c57c219e4dd346e44a
Fixes: #3924

3 files changed, 11 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index 9bb0eee43..08f23c4af 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -114,6 +114,9 @@ class SchemaItem(SchemaEventTarget, visitors.Visitable):
         schema_item.dispatch._update(self.dispatch)
         return schema_item
 
+    def _translate_schema(self, effective_schema, map_):
+        return map_.get(effective_schema, effective_schema)
+
 
 class Table(DialectKWArgs, SchemaItem, TableClause):
     r"""Represent a table in a database.
@@ -3974,7 +3977,8 @@ class _SchemaTranslateMap(object):
         if map_ is not None:
             def schema_for_object(obj):
                 effective_schema = self._default_schema_getter(obj)
-                effective_schema = map_.get(effective_schema, effective_schema)
+                effective_schema = obj._translate_schema(
+                    effective_schema, map_)
                 return effective_schema
             self.__call__ = schema_for_object
             self.hash_key = ";".join(
diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index 91a12bd33..b69d667c6 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -351,6 +351,9 @@ class FromClause(Selectable):
 
     """
 
+    def _translate_schema(self, effective_schema, map_):
+        return effective_schema
+
     _memoized_property = util.group_expirable_memoized_property(["_columns"])
 
     @util.deprecated(
diff --git a/lib/sqlalchemy/sql/sqltypes.py b/lib/sqlalchemy/sql/sqltypes.py
index 78a130f62..bb39388ab 100644
--- a/lib/sqlalchemy/sql/sqltypes.py
+++ b/lib/sqlalchemy/sql/sqltypes.py
@@ -997,6 +997,9 @@ class SchemaType(SchemaEventTarget):
                 util.portable_instancemethod(self._on_metadata_drop)
             )
 
+    def _translate_schema(self, effective_schema, map_):
+        return map_.get(effective_schema, effective_schema)
+
     def _set_parent(self, column):
         column._on_table_attach(util.portable_instancemethod(self._set_table))