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

Base = declarative_base(create_engine('postgres://scott:tiger@127.0.0.1/test', echo=True))

class Parent(Base):
    __tablename__ = 'parent'
    
    id  = Column(Integer, primary_key=True)
    data = Column(String)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relation("Child", cascade="all, delete", passive_deletes=True)
    
class Child(Base):
    __tablename__ = 'child'

    id  = Column(Integer, primary_key=True)
    data = Column(String)


Base.metadata.drop_all()
Base.metadata.create_all()

s = create_session()

p1 = Parent(data='p1', child=Child(data='c1'))
s.add(p1)
s.flush()

s.clear()

p1 = s.query(Parent).get(p1.id)
s.delete(p1)
s.flush()