SQLAlchemy Generic Relationship simple example

Question

I know similar questions have been asked, however I am really struggling to understand how generic fields are implemented in SQLAlchemy.

I have a Permissions class/table which I want to contain a field which can relate to any model type.

I have looked at the examples and this blog post http://techspot.zzzeek.org/2007/05/29/polymorphic-associations-with-sqlalchemy/

Is it possible to have a generic relation without a separate table? Just by storing the object_type and id? Something along these lines:

class Permission(AbstractBase):
    user = relationship("User", backref=backref('permissions'))
    permission_type = column(String())
    object = #The object the permission applies to, could be any type.

I guess just a really simple example would be appreciated!

Also, it's worth noting I am coming from a Django background!

Thanks

Answer 1

You can also use generic_relationship from the sqlalchemy-utils package.

Here's an example for their documentation;

from sqlalchemy_utils import generic_relationship

class User(Base):
    __tablename__ = 'user'
    id = sa.Column(sa.Integer, primary_key=True)

class Customer(Base):
    __tablename__ = 'customer'
    id = sa.Column(sa.Integer, primary_key=True)

class Event(Base):
    __tablename__ = 'event'
    id = sa.Column(sa.Integer, primary_key=True)
    object_type = sa.Column(sa.Unicode(255))
    object_id = sa.Column(sa.Integer)
    object = generic_relationship(object_type, object_id)

Answer 2

Someone else asked the identical question about a day later, I pointed to the three examples we have for this kind of thing but also I wrote a new example that will do (mostly) the same thing Django does (minus that weird "contenttypes" table): sqlalchemy generic foreign key (like in django ORM)