sqlalchemy：插入连接表

Question

我想在一个使用联合继承的表中添加一条记录。出于某种我无法理解的原因，依赖的表没有被插入数据。

## Inheritance test case.
from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import Session, relationship, backref
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()
class Employee(Base):
    __tablename__ = 'personnel'
    __mapper_args__ = { 'polymorphic_on': 'etyp' }

    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    etyp = Column(String(10))

class Engineer(Employee):
    __mapper_args__ = { 'polymorphic_identity':'eng' }
    __tablename__ = "engineers"
    id = Column(Integer, ForeignKey(Employee.id), index=True)
    eng_data = Column(String(50))

engine = create_engine('sqlite:///', echo=True)
Base.metadata.create_all(engine)
session = Session(bind=engine)

e1 = Engineer(name="wally", eng_data="lazy guy")

session.add(e1)
session.commit()
# note that the Engineer table is not INSERTed into

e1 = session.query(Employee).filter_by(name="wally").one()
# the next line triggers an exception because it tries to lookup
# the Engineer row, which is missing
print e1.eng_data

这是在使用Python 2.7和sqlalchemy 0.9.4（Debian测试版）进行的。

Answer 1

把 engineers.id 设置为 主键 就能解决这个问题：

class Engineer(Employee):
    # ...
    id = Column(Integer, ForeignKey(Employee.id), primary_key=True)