[Sqlalchemy-tickets] Issue #4110: "AttributeError: parent" when using association_proxy on polymorphic mapping (zzzeek/sqlalchemy)

[Kai K. <iss...@bi...>]

New issue 4110: "AttributeError: parent" when using association_proxy on polymorphic mapping
https://bitbucket.org/zzzeek/sqlalchemy/issues/4110/attributeerror-parent-when-using

Kai Kölger:

MCVE:

```
#!python


from sqlalchemy import create_engine, Integer

from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base

from sqlalchemy.orm import sessionmaker,  relationship
from sqlalchemy.schema import Column, ForeignKey

Session = sessionmaker()
engine = create_engine('sqlite:///:memory:')
Session.configure(bind=engine)
Base = declarative_base()


_session = Session()


class A1(Base):
    __tablename__ = 'a1'

    id = Column(Integer, primary_key=True)
    obj_type = Column(Integer)

    __mapper_args__ = {
        'polymorphic_on': obj_type,
        'polymorphic_identity': 1,
    }


class ARef(Base):
    __tablename__ = 'aref'

    id = Column(Integer, primary_key=True)

    referer_id = Column(Integer, ForeignKey('a1.id'))

    refered_id = Column(Integer, ForeignKey('a1.id'))
    refered = relationship(
       'A1', uselist=False, primaryjoin='a1.id == aref.refered_id')


class A2(A1):
    __mapper_args__ = {'polymorphic_identity': 2}


class A3(A1):
    __mapper_args__ = {'polymorphic_identity': 3}

    _a2_ref = relationship('ARef')
    a2_list = association_proxy(_a2_ref, 'refered')


Base.metadata.create_all(engine)

```

The goal was to have a collection of references between some of the derived classes of A1.

The MCVE produces with sqlalchemy 1.1.14 this stack trace:

```
#!
Traceback (most recent call last):
  File "_experiment5.py", line 47, in <module>
    class A3(A1):
  File "_experiment5.py", line 51, in A3
    a2_list = association_proxy(_a2_ref, 'refered')
  File "/home/kk/ve/arc2/lib/python3.4/site-packages/sqlalchemy/ext/associationproxy.py", line 76, in association_proxy
    return AssociationProxy(target_collection, attr, **kw)
  File "/home/kk/ve/arc2/lib/python3.4/site-packages/sqlalchemy/ext/associationproxy.py", line 156, in __init__
    type(self).__name__, target_collection, id(self))
  File "/home/kk/ve/arc2/lib/python3.4/site-packages/sqlalchemy/orm/relationships.py", line 1445, in __str__
    return str(self.parent.class_.__name__) + "." + self.key
  File "/home/kk/ve/arc2/lib/python3.4/site-packages/sqlalchemy/util/langhelpers.py", line 850, in __getattr__
    return self._fallback_getattr(key)
  File "/home/kk/ve/arc2/lib/python3.4/site-packages/sqlalchemy/util/langhelpers.py", line 828, in _fallback_getattr
    raise AttributeError(key)
AttributeError: parent

```