How can I re-use a sqlalchemy ORM model on multiple databases and schemas?

Question

I have a SQLAlchemy ORM model that currently looks a bit like this:

Base = declarative_base()

class Database(Base):

  __tablename__ = "databases"
  __table_args__ = (
    saschema.PrimaryKeyConstraint('db', 'role'),
    {
      'schema' : 'defines',
      },
    )


  db = Column(String, nullable=False)
  role = Column(String, nullable=False)
  server = Column(String)

Here's the thing, in practice this model exists in multiple databases, and in those databases it'll exist in mutiple schemas. For any one operation I'll only use one (database, schema) tuple.

Right now, I can set the database engine using this:

Session = scoped_session(sessionmaker())
Session.configure(bind=my_db_engine)
# ... do my operations on the model here.

But I'm not sure how I can change the __table_args__ at execution time so that the schema will be the right one.