[Sqlalchemy-tickets] Issue #4153: Eager loading of specific polymorphic subtypes triggers errors in lazy loaded relationships (zzzeek/sqlalchemy)

[Theron L. <iss...@bi...>]

New issue 4153: Eager loading of specific polymorphic subtypes triggers errors in lazy loaded relationships
https://bitbucket.org/zzzeek/sqlalchemy/issues/4153/eager-loading-of-specific-polymorphic

Theron Luhn:

Congrats on 1.2.0 and thanks for your hard work!  I'm excited to upgrade and especially looking forward to selectin loading.

While attempting to upgrade, I'm getting errors caused by eager loading on polymorphic subtypes (the loading strategy detailed [here](http://docs.sqlalchemy.org/en/latest/orm/inheritance_loading.html#eager-loading-of-specific-or-polymorphic-subtypes)).  It was very difficult to pin down, but I finally am able to make a reproducible test case:


```
#!python

from sqlalchemy import String, Integer, Column, create_engine, ForeignKey, inspect
from sqlalchemy.orm import relationship, Session, subqueryload
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


class Company(Base):
    __tablename__ = 'company'
    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    employees = relationship('Employee')
    slogans = relationship('Slogan')


class Slogan(Base):
    __tablename__ = 'slogan'
    id = Column(Integer, primary_key=True)
    text = Column(String)
    company_id = Column(Integer, ForeignKey('company.id'), nullable=False)


class Employee(Base):
    __tablename__ = 'employee'
    id = Column(Integer, primary_key=True)
    type = Column(String, nullable=False)
    name = Column(String, nullable=False)
    company_id = Column(Integer, ForeignKey('company.id'), nullable=False)

    __mapper_args__ = {
        'polymorphic_on': 'type',
        'with_polymorphic': '*',
    }


class Programmer(Employee):
    __tablename__ = 'programmer'
    id = Column(Integer, ForeignKey('employee.id'), primary_key=True)
    languages = relationship('Language')

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


class Language(Base):
    __tablename__ = 'language'
    id = Column(Integer, primary_key=True)
    programmer_id = Column(
        Integer,
        ForeignKey('programmer.id'),
        nullable=False,
    )
    name = Column(String, nullable=False)


engine = create_engine('postgresql://localhost:5432/sa')
Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)
db = Session(engine)


company = Company(
    id=1,
    name='Foobar Corp',
    employees=[Programmer(
        id=1,
        name='John Smith',
        languages=[Language(id=1, name='Python')],
    )],
    slogans=[Slogan(
        id=1,
        text='Because I said so.',
    )],
)
db.add(company)
db.flush()
db.expunge_all()

company = db.query(Company).filter(
    Company.id == 1,
).options(
    subqueryload(Company.employees.of_type(Programmer)).subqueryload(Programmer.languages),
).one()
print(company.employees)  # Eager-loaded relationships work
print(company.slogans)  # Any lazy-loaded relationships trigger error

```

This script triggers the following error:

```
Traceback (most recent call last):
  File "polybug.py", line 87, in <module>
    print(company.slogans)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 242, in __get__
    return self.impl.get(instance_state(instance), dict_)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 603, in get
    value = self.callable_(state, passive)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/strategies.py", line 623, in _load_for_state
    return self._emit_lazyload(session, state, ident_key, passive)
  File "<string>", line 1, in <lambda>
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/strategies.py", line 703, in _emit_lazyload
    state.load_options, effective_path
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/ext/baked.py", line 178, in _add_lazyload_options
    cache_key = opt._generate_cache_key(cache_path)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/strategy_options.py", line 404, in _generate_cache_key
    c_key = opt._generate_cache_key(path)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/strategy_options.py", line 93, in _generate_cache_key
    for token in chopped:
TypeError: 'NoneType' object is not iterable
```

This is running on Python 3.6.2 with SQLAlchemy 1.2.0.  SQLAlchemy 1.1.10 behaves as expected and does not trigger an error.

I'm using psycopg2 v2.7.3.2 and Postgres 10.1