[Sqlalchemy-tickets] Issue #3462: new single inh polymorphic identity logic in query should skip if there's no actual identity (zzzeek/sqlalchemy)

[Mike B. <iss...@bi...>]

New issue 3462: new single inh polymorphic identity logic in query should skip if there's no actual identity
https://bitbucket.org/zzzeek/sqlalchemy/issue/3462/new-single-inh-polymorphic-identity-logic

Mike Bayer:

e.g.


```
#!python

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class Client(Base):
    __tablename__ = 'client'
    id = Column(Integer, primary_key=True)


class SubClient(Client):
    pass


class Thing(Base):
    __tablename__ = 'thing'
    id = Column(Integer, primary_key=True)
    cid = Column(ForeignKey('client.id'))

s = Session()

print s.query(Thing).join(SubClient, Thing.cid == SubClient.id)


```

produces:

```
#!sql


SELECT thing.id AS thing_id, thing.cid AS thing_cid 
FROM thing JOIN client ON thing.cid = client.id AND NULL
```