https://stackoverflow.com/questions/29442635/sqlalchemy-dele

#!/usr/bin/env python
import contextlib

import sqlalchemy
import sqlalchemy.ext.declarative
import sqlalchemy.orm
import sqlalchemy.exc

exc = sqlalchemy.exc

from sqlalchemy import (
    Column,
    ForeignKey,
    Index,
    Integer,
    Numeric,
)
from sqlalchemy.orm import (
    relationship,
)

Base = sqlalchemy.ext.declarative.declarative_base()

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
class ORDER(Base):
    __tablename__ = 'ORDERS'

    ORDERSID = Column(Integer, primary_key=True)
    ORDERSCOST = Column(Numeric(19, 4), nullable=False)

    # ORDEREDITEM = relationship("ORDEREDITEM")
    ORDEREDITEM = relationship("ORDEREDITEM", cascade="all,delete")

class ORDEREDITEM(Base):
    __tablename__ = 'ORDEREDITEMS'
    __table_args__ = (
        Index('AK_ORDERSID_ITEMID', 'ORDERSID', 'ITEMSID', unique=True),
    )

    ORDEREDITEMSID = Column(Integer, primary_key=True)
    # ITEMSID = Column(ForeignKey(u'ITEMS.ITEMSID'), nullable=False, index=True)
    ITEMSID = Column(Integer, nullable=False, index=True)
    ORDERSID = Column(ForeignKey(u'ORDERS.ORDERSID', ondelete=u'CASCADE'), nullable=False)
    ORDEREDITEMSQUANTITY = Column(Integer, nullable=False)

    ORDER = relationship(u'ORDER')

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
engine = sqlalchemy.create_engine('sqlite:///:memory:', echo=False)
Base.metadata.create_all(engine)

Session = sqlalchemy.orm.sessionmaker(bind=engine)
DBSession = Session

def raise_database_error(error):
    print('db error', error)

def main():
    orderID = 1

    order = ORDER(
        ORDERSID = orderID,
        ORDERSCOST = 20.0,
    )
    item = ORDEREDITEM(
        ORDEREDITEMSID = 1,
        ITEMSID = 1,
        ORDERSID = orderID,
        ORDEREDITEMSQUANTITY = 2,
    )
    with contextlib.closing(DBSession()) as session:
        session.add(order)
        session.add(item)
        session.commit()

        order = session.query(ORDER).filter_by(ORDERSID=orderID).first()
        assert order is not None

        item = session.query(ORDEREDITEM).filter_by(ORDEREDITEMSID=1).first()
        assert item is not None

    with contextlib.closing(DBSession()) as session:
        try:
            returnedOrder = session.query(ORDER).filter_by(ORDERSID=orderID).first()
            session.delete(returnedOrder)
            session.commit()
        except exc.SQLAlchemyError as error:
            session.rollback()
            raise_database_error(error)
        # else:
        #     return '1'

    with contextlib.closing(DBSession()) as session:
        order = session.query(ORDER).filter_by(ORDERSID=orderID).first()
        assert order is None

        item = session.query(ORDEREDITEM).filter_by(ORDEREDITEMSID=1).first()
        assert item is None

if __name__ == '__main__':
    main()