[Sqlalchemy-tickets] Issue #3219: Intermediate __abstract__ inheriting from model doesn't transfer model's attrs (zzzeek/sqlalchemy)

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

New issue 3219: Intermediate __abstract__ inheriting from model doesn't transfer model's attrs
https://bitbucket.org/zzzeek/sqlalchemy/issue/3219/intermediate-__abstract__-inheriting-from

David Lord:

I have the following models:

* `Device`
* `DeviceSource` has a foreign key and relationship to `Device`
* `LDAPDeviceSource` is a joined-table inheritance of `DeviceSource`

In order to make defining new `DeviceSource` subclasses easier, I created `DeviceSourceMixin`, which inherits from `DeviceSource` and provides declared attrs for `__tablename__` and the foreign primary `id`.  `DeviceSourceMixin` is `__abstract__` so that it doesn't create a table of its own.

The issue is that this intermediate `__abstract__` seems to break the declarative model.  The foreign key and relationship in `DeviceSource` do not get inherited by `LDAPDeviceSource` when subclassing `DeviceSourceMixin`.

This code demonstrates the issue:

```
import sqlalchemy as sa
from sqlalchemy.ext.declarative import declared_attr, as_declarative
from sqlalchemy.orm import Session, relationship

engine = sa.create_engine('sqlite:///:memory:', echo=True)
session = Session(bind=engine)


@as_declarative(bind=engine)
class Base(object):
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

    id = sa.Column(sa.Integer, primary_key=True)


class Device(Base):
    pass


class DeviceSource(Base):
    type = sa.Column(sa.String, nullable=False)
    device_id = sa.Column(sa.Integer, sa.ForeignKey(Device.id), nullable=False)

    device = relationship(Device, backref='sources')

    __mapper_args__ = {
        'polymorphic_on': type
    }


class DeviceSourceMixin(DeviceSource):
    __abstract__ = True

    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

    @declared_attr
    def id(cls):
        return sa.Column(sa.Integer, sa.ForeignKey(DeviceSource.id), primary_key=True)


class LDAPDeviceSource(DeviceSourceMixin):
    name = sa.Column(sa.String, nullable=False)

    __mapper_args__ = {
        'polymorphic_identity': 'ldap'
    }


Base.metadata.create_all()
d1 = Device()
s1 = LDAPDeviceSource(device=d1, name='s1')
session.add(s1)
session.commit()
```

It produces the following error:

```
Traceback (most recent call last):
  File "/home/david/Projects/cedar/example2.py", line 62, in <module>
    s1 = LDAPDeviceSource(device=d1, name='s1')
  File "<string>", line 4, in __init__
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/state.py", line 260, in _initialize_instance
    return manager.original_init(*mixed[1:], **kwargs)
  File "<string>", line 6, in __init__
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/ext/declarative/base.py", line 526, in _declarative_constructor
    setattr(self, k, kwargs[k])
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 226, in __set__
    instance_dict(instance), value, None)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 812, in set
    value = self.fire_replace_event(state, dict_, value, old, initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 832, in fire_replace_event
    state, value, previous, initiator or self._replace_token)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 1148, in emit_backref_from_scalar_set_event
    passive=PASSIVE_NO_FETCH)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 980, in append
    collection.append_with_event(value, initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 653, in append_with_event
    self._data()._sa_appender(item, _sa_initiator=initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 1047, in append
    item = __set(self, item, _sa_initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 1019, in __set
    item = executor.fire_append_event(item, _sa_initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 716, in fire_append_event
    item, initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 929, in fire_append_event
    value = fn(state, value, initiator or self._append_token)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 1157, in emit_backref_from_collection_append_event
    child_impl = child_state.manager[key].impl
KeyError: 'device'
```