_ColumnEntity selectable needs to be consistent with its mapper zero

from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

class Locality(Base):
    Id = Column( Integer, primary_key = True, autoincrement = True)
    __tablename__ = 'locality'
    name = Column( String(128), nullable = False)

class Street(Base):
    Id = Column(Integer, primary_key = True)
    __tablename__ = 'street'
    name = Column( String(128), nullable = False)
    localityId = Column(Integer, ForeignKey(Locality.Id))
    locality = orm.relation(Locality, primaryjoin = localityId == Locality.Id)

session.query(Locality.name + ' ' + Street.name).join(Street.locality)

# vs

session.query(Street.name + ' ' + Locality.name).join(Street.locality)

# in one case, the ent.selectable is used, in the other, fallback to "left".
# ent.selectable needs to be consistent with mapper_zero on _ColumnElement

patch:

diff -r e30d02c6259d5a2386fd836e27dee75e3a143bd2 lib/sqlalchemy/orm/query.py
--- a/lib/sqlalchemy/orm/query.py   Thu Jun 16 12:09:45 2011 -0400
+++ b/lib/sqlalchemy/orm/query.py   Mon Jun 20 10:33:17 2011 -0400
@@ -2845,7 +2845,8 @@

     def setup_entity(self, entity, mapper, adapter, from_obj,
                                 is_aliased_class, with_polymorphic):
-        self.selectable = from_obj
+        if 'selectable' not in self.__dict__:
+            self.selectable = from_obj
         self.froms.add(from_obj)

     def corresponds_to(self, entity):

test should include:

the above joins
a variety of _ColumnElement ensure that selectable matches to mapper_zero, try to get aliasing in there

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