- mapping to a select() construct now requires that you

make an alias() out of it distinctly.   This to eliminate
confusion over such issues as [ticket:1542]

1 files changed, 4 insertions, 5 deletions

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index 13151f232..a9dad9176 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -163,11 +163,10 @@ class Mapper(object):
                 self.with_polymorphic = None
 
         if isinstance(self.local_table, expression._SelectBaseMixin):
-            util.warn("mapper %s creating an alias for the given "
-                        "selectable.  References to the original selectable "
-                        "may be misinterpreted by queries, polymorphic_on, etc. "
-                        " Consider passing an explicit selectable.alias() construct instead." % self)
-            self.local_table = self.local_table.alias()
+            raise sa_exc.InvalidRequestError(
+                "When mapping against a select() construct, map against an alias() of the construct instead."
+                "This because several databases don't allow a SELECT from a subquery that does not have an alias."
+                )
 
         if self.with_polymorphic and isinstance(self.with_polymorphic[1], expression._SelectBaseMixin):
             self.with_polymorphic = (self.with_polymorphic[0], self.with_polymorphic[1].alias())