unexpected/inconsistent aliasing applied to single-table inh subclass in query.join()

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

Base = declarative_base()


class A(Base):
    __tablename__ = "a"

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

    __mapper_args__ = {
        'polymorphic_on': type,
        'polymorphic_identity': 'a'
    }


class ASub1(A):
    __mapper_args__ = {
        'polymorphic_identity': 'asub1'
    }


class ASub2(A):
    __mapper_args__ = {
        'polymorphic_identity': 'asub2'
    }


class B(Base):
    __tablename__ = 'b'

    id = Column(Integer, primary_key=True)

    a1_id = Column(Integer, ForeignKey("a.id"))
    a2_id = Column(Integer, ForeignKey("a.id"))

    a1 = relationship("A", primaryjoin="B.a1_id == A.id", backref='b1')
    a2 = relationship("A", primaryjoin="B.a2_id == A.id", backref='b2')

s = Session()

print s.query(ASub1).join(B, ASub1.b1).join(ASub2, B.a2)

print s.query(ASub1).join(B, ASub1.b1).join(ASub2, ASub2.id == B.a2_id)

first query:

SELECT a.id AS a_id, a.type AS a_type 
FROM a JOIN b ON b.a1_id = a.id JOIN a ON b.a2_id = a.id AND a.type IN (:type_1) 
WHERE a.type IN (:type_2)

second query, it's automatigically aliasing ASub2, bizarrely without the relationship:

SELECT a.id AS a_id, a.type AS a_type 
FROM a JOIN b ON b.a1_id = a.id JOIN a AS a_1 ON a_1.id = b.a2_id AND a_1.type IN (:type_1) 
WHERE a_1.type IN (:type_2)

this is really bad, is this happening with joined inheritance, whats the rules for the magic aliasing here? changing this is going to be really backwards incompatible.

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