collection.py

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy

Base = declarative_base()

class Machine(Base):
    __tablename__ = 'machines'
    __table_args__ = {'sqlite_autoincrement': True}
    
    machine_id = Column(Integer, primary_key=True)

    widgets = relationship('Widget')


class Widget(Base):
    __tablename__ = 'widgets'
    __table_args__ = {'sqlite_autoincrement': True}

    widget_id = Column(Integer, primary_key=True)
    machine_id = Column(Integer, ForeignKey('machines.machine_id'), nullable=False)

    _gadgets = relationship('Gadget')

    gadgets = association_proxy('_gadgets', 'json_repr',
                               creator=lambda kwargs: Gadget(**kwargs))
    

class Gadget(Base):
    __tablename__ = 'gadgets'
    __table_args__ = {'sqlite_autoincrement': True}

    gadget_id = Column(Integer, primary_key=True)
    widget_id = Column(Integer, ForeignKey('widgets.widget_id'), nullable=False)
    machine_id = Column(Integer, nullable=False)

    a = Column(String)
    b = Column(String)
    c = Column(Integer)

    @property
    def json_repr(self):
        return dict(a=self.a, b=self.b, c=self.c)


if __name__ == '__main__':
    engine = create_engine('sqlite:///:memory:', echo=True)
    Base.metadata.bind = engine
    Base.metadata.create_all()
    session = sessionmaker(bind=engine)()

    m = Machine()
    w = Widget()

    session.add(m)
    m.widgets.append(w)
    w.gadgets.append(dict(a='1', b='2'))
    session.commit()