Data Modeling, Flask-SQLAlchemy and Plugins
Extending the class and adding an .update() method is just how it is supposed to work for option 2.I don't see any problem in it (and i'm using that style of coding with flask/sqlalchemy all the time).
And if you (can) omit the dynamic lazy attribute you could also do a thing like:
self.datapieces.append(new_datapiece)in your Feed's update function.