- Two fixes to help prevent out-of-band columns from

being rendered in polymorphic_union inheritance
scenarios (which then causes extra tables to be
rendered in the FROM clause causing cartesian
products):
- improvements to "column adaption" for
  a->b->c inheritance situations to better
  locate columns that are related to one
  another via multiple levels of indirection,
  rather than rendering the non-adapted
  column.
- the "polymorphic discriminator" column is
  only rendered for the actual mapper being
  queried against. The column won't be
  "pulled in" from a subclass or superclass
  mapper since it's not needed.

2 files changed, 27 insertions, 14 deletions

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index b48297a64..48c2f9e27 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -612,7 +612,13 @@ class Mapper(object):
                 # right set
                 if hasattr(self, '_cols_by_table') and col.table in self._cols_by_table and col not in self._cols_by_table[col.table]:
                     self._cols_by_table[col.table].add(col)
-
+            
+            # if this ColumnProperty represents the "polymorphic discriminator"
+            # column, mark it.  We'll need this when rendering columns
+            # in SELECT statements.
+            if not hasattr(prop, '_is_polymorphic_discriminator'):
+                prop._is_polymorphic_discriminator = (col is self.polymorphic_on or prop.columns[0] is self.polymorphic_on)
+                
             self.columns[key] = col
             for col in prop.columns:
                 for col in col.proxy_set:
@@ -860,20 +866,27 @@ class Mapper(object):
         else:
             return mappers, self._selectable_from_mappers(mappers)
 
-    @property
-    def _default_polymorphic_properties(self):
-        return util.unique_list(
-            chain(*[list(mapper.iterate_properties) for mapper in [self] + self._with_polymorphic_mappers])
-        )
-        
     def _iterate_polymorphic_properties(self, mappers=None):
+        """Return an iterator of MapperProperty objects which will render into a SELECT."""
+        
         if mappers is None:
-            return iter(self._default_polymorphic_properties)
+            mappers = self._with_polymorphic_mappers
+
+        if not mappers:
+            for c in self.iterate_properties:
+                yield c
         else:
-            return iter(util.unique_list(
+            # in the polymorphic case, filter out discriminator columns
+            # from other mappers, as these are sometimes dependent on that
+            # mapper's polymorphic selectable (which we don't want rendered)
+            for c in util.unique_list(
                 chain(*[list(mapper.iterate_properties) for mapper in [self] + mappers])
-            ))
-
+            ):
+                if getattr(c, '_is_polymorphic_discriminator', False) and \
+                    (not self.polymorphic_on or c.columns[0] is not self.polymorphic_on):
+                        continue
+                yield c
+    
     @property
     def properties(self):
         raise NotImplementedError("Public collection of MapperProperty objects is "
diff --git a/lib/sqlalchemy/sql/util.py b/lib/sqlalchemy/sql/util.py
index d5f2417c2..1bd4dd857 100644
--- a/lib/sqlalchemy/sql/util.py
+++ b/lib/sqlalchemy/sql/util.py
@@ -439,12 +439,12 @@ class ClauseAdapter(visitors.ReplacingCloningVisitor):
         self.exclude = exclude
         self.equivalents = equivalents or {}
         
-    def _corresponding_column(self, col, require_embedded):
+    def _corresponding_column(self, col, require_embedded, _seen=util.EMPTY_SET):
         newcol = self.selectable.corresponding_column(col, require_embedded=require_embedded)
 
-        if not newcol and col in self.equivalents:
+        if not newcol and col in self.equivalents and col not in _seen:
             for equiv in self.equivalents[col]:
-                newcol = self.selectable.corresponding_column(equiv, require_embedded=require_embedded)
+                newcol = self._corresponding_column(equiv, require_embedded=require_embedded, _seen=_seen.union([col]))
                 if newcol:
                     return newcol
         return newcol