[Sqlalchemy-tickets] Issue #3185: More than one level of abstract concrete base classes does not work (zzzeek/sqlalchemy)

[Alex G. <iss...@bi...>]

New issue 3185: More than one level of abstract concrete base classes does not work
https://bitbucket.org/zzzeek/sqlalchemy/issue/3185/more-than-one-level-of-abstract-concrete

Alex Grönholm:

The following code fails with AttributeError: 'NoneType' object has no attribute 'concrete':
```
#!python

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

Base = declarative_base()


class Document(Base, AbstractConcreteBase):
    doctype = Column(Unicode, nullable=False)


class ContactDocument(Document):
    __abstract__ = True

    send_method = Column('sendmethod', Unicode)


class ActualDocument(ContactDocument):
    __tablename__ = 'actual_documents'
    __mapper_args__ = {'concrete': True, 'polymorphic_identity': 'actual'}

    id = Column(Integer, primary_key=True)

configure_mappers()
```

This is caused by the __declare_first__() in AbstractConcreteBase not checking if the target subclass has a mapper or not. Attached is the patch I was given to remedy the issue.