[Sqlalchemy-tickets] Issue #4287: baked cache key generation fails on relationship from subclass (zzzeek/sqlalchemy)

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

New issue 4287: baked cache key generation fails on relationship from subclass
https://bitbucket.org/zzzeek/sqlalchemy/issues/4287/baked-cache-key-generation-fails-on

Michael Bayer:

```
#!python

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

Base = declarative_base()


class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)


class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship('Parent', backref=backref('children'))

    type = Column(String(), nullable=False)
    __mapper_args__ = {
        'polymorphic_on': type,
    }


class ChildSubclass1(Child):
    __tablename__ = 'child_subclass1'
    id = Column(Integer, ForeignKey('child.id'), primary_key=True)
    __mapper_args__ = {
        'polymorphic_identity': 'subclass1',
        'polymorphic_load': 'selectin'
    }


class Other(Base):
    __tablename__ = 'other'

    id = Column(Integer, primary_key=True)
    child_subclass_id = Column(Integer, ForeignKey('child_subclass1.id'))
    child_subclass = relationship('ChildSubclass1', backref=backref('others'))

configure_mappers()

opt = joinedload(
    Parent.children.of_type(ChildSubclass1)
).joinedload(ChildSubclass1.others)

path = Parent.__mapper__._path_registry[Parent.children.property][Child.__mapper__]
print(
    opt._generate_cache_key(path)
)

```

generates:


```
#!python

((<class '__main__.ChildSubclass1'>, ('lazy', 'joined')),)

```

e.g. not taking "Other" into account.   Load._chop_path() is failing.  See also #4286 where this was found.