[Sqlalchemy-tickets] Issue #4241: selectin loading in Single Table Inheritance produces cross join (zzzeek/sqlalchemy)

[Tim C. <iss...@bi...>]

New issue 4241: selectin loading in Single Table Inheritance produces cross join
https://bitbucket.org/zzzeek/sqlalchemy/issues/4241/selectin-loading-in-single-table

Tim Chen:

```
#!python

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

Base = declarative_base()


class User(Base):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    type = Column(String)

    __mapper_args__ = {
        'polymorphic_on': type,
    }


class EmployerUser(User):
    # __tablename__ = 'employer_user'

    # id = Column(Integer, ForeignKey('user.id'), primary_key=True)
    roles = relationship('Role', lazy='selectin')

    __mapper_args__ = {
        'polymorphic_identity': 'employer',
    }


class Role(Base):
    __tablename__ = 'role'

    id = Column(Integer, primary_key=True)
    user_id = Column(Integer, ForeignKey('user.id'))


e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)

user = EmployerUser(roles=[Role(), Role(), Role()])
s.add(user)
s.commit()

s.query(EmployerUser).all()

```

The generated selectin load SQL looks like this:
```
#!sql

SELECT user_1.id AS user_1_id, role.id AS role_id, role.user_id AS role_user_id
FROM user, user AS user_1 JOIN role ON user_1.id = role.user_id
WHERE user_1.id IN (?) AND user.type IN (?) AND user_1.type IN (?) ORDER BY user_1.id
```

You can see that user gets cross joined to itself, blowing up the result set unnecessarily.  This becomes a huge problem for large tables.