- 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]

4 files changed, 16 insertions, 12 deletions

diff --git a/CHANGES b/CHANGES
index e02f56954..e67373262 100644
--- a/CHANGES
+++ b/CHANGES
@@ -31,6 +31,10 @@ CHANGES
 
   - the "named tuple" objects returned when iterating a
     Query() are now pickleable.
+
+  - 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]
     
   - query.join() has been reworked to provide more consistent 
     behavior and more flexibility (includes [ticket:1537])
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())
diff --git a/test/orm/test_mapper.py b/test/orm/test_mapper.py
index 79286fe2a..321ac25c1 100644
--- a/test/orm/test_mapper.py
+++ b/test/orm/test_mapper.py
@@ -115,12 +115,9 @@ class MapperTest(_fixtures.FixtureTest):
         s = sa.select([users.c.name]).alias('foo')
         assert_raises(sa.exc.ArgumentError, mapper, User, s)
 
-    @testing.emits_warning(
-        'mapper Mapper|User|Select object creating an alias for '
-        'the given selectable - use Class attributes for queries')
     @testing.resolve_artifact_names
     def test_no_pks_2(self):
-        s = sa.select([users.c.name])
+        s = sa.select([users.c.name]).alias()
         assert_raises(sa.exc.ArgumentError, mapper, User, s)
 
     @testing.resolve_artifact_names
diff --git a/test/orm/test_selectable.py b/test/orm/test_selectable.py
index bfa400895..e46d8bbc8 100644
--- a/test/orm/test_selectable.py
+++ b/test/orm/test_selectable.py
@@ -27,15 +27,19 @@ class SelectableNoFromsTest(_base.MappedTest):
     @testing.resolve_artifact_names
     def test_no_tables(self):
 
-        selectable = select(["x", "y", "z"])
+        selectable = select(["x", "y", "z"]).alias()
         assert_raises_message(sa.exc.InvalidRequestError,
                                  "Could not find any Table objects",
                                  mapper, Subset, selectable)
 
-    @testing.emits_warning('.*creating an Alias.*')
     @testing.resolve_artifact_names
-    def test_basic(self):
+    def test_no_selects(self):
         subset_select = select([common.c.id, common.c.data])
+        assert_raises(sa.exc.InvalidRequestError, mapper, Subset, subset_select)
+        
+    @testing.resolve_artifact_names
+    def test_basic(self):
+        subset_select = select([common.c.id, common.c.data]).alias()
         subset_mapper = mapper(Subset, subset_select)
 
         sess = create_session(bind=testing.db)