[sqlalchemy] Mixing a column declared with @declared_attr into a child class is not mapped in parent class when using STI

[Amos]

I am attempting to use STI with mixins on the derived/inheriting
classes. If the mixin has a simple Column, everything works great. But
when I have a ForeignKey on the Column, and therefore use the
`declared_attr` decorator, I get an exception when I attempt to query
the child collection.

For the example below, querying the Engineer class throws the
following exception:

InvalidRequestError: Class <class 'Person'> does not have a mapped
column named 'target_id'

Here's some code:

    class Person(Base):
        id = Column(Integer, primary_key=True)
        type = Column('type', String(50))

        @declared_attr
        def __tablename__(cls):
            if has_inherited_table(cls):
                return None
            return cls.__name__.lower()

        @declared_attr
        def __mapper_args__(cls):
            if cls.__name__ == 'Person':
                mapper_args = {}
                mapper_args['polymorphic_on'] = cls.type
                return mapper_args
            else:
                return {"polymorphic_identity": cls.__name__}

    class Mixin(object):
        simple_column = Column(Integer, nullable=True) # This column
is mapped correctly

        @declared_attr
        def target_id(cls): # But when I have a column with a
ForeignKey, and I used `declared_attr` - everything breaks down
            return Column('target_id', ForeignKey('target.id'))

        @declared_attr
        def target(cls): # Relationships also work correctly, if
target_id is defined in the Person class
            return relationship("Target")

    class Engineer(Person, Mixin):
        pass

I know I can get around this by declaring the target_id on the Person
class but then I lose the encapsulation provided by the Mixin class.
Why does declaring a simple Column on the Mixin work perfectly, but
fail when declaring a column using the `declared_attr` decorator?

-- 
You received this message because you are subscribed to the Google Groups 
"sqlalchemy" group.
To post to this group, send email to sqlalchemy@googlegroups.com.
To unsubscribe from this group, send email to 
sqlalchemy+unsubscr...@googlegroups.com.
For more options, visit this group at 
http://groups.google.com/group/sqlalchemy?hl=en.