如何在sqlalchemy中加入同一个表

Question

我正在尝试加入sqlalchemy中的同一个表.这是我尝试过的最小版本:

#!/usr/bin/env python
import sqlalchemy as sa
from sqlalchemy import create_engine
from sqlalchemy.orm import mapper, sessionmaker, aliased

engine = create_engine('sqlite:///:memory:', echo=True)

metadata = sa.MetaData()
device_table = sa.Table("device", metadata,
    sa.Column("device_id", sa.Integer, primary_key=True),
    sa.Column("name", sa.String(255), nullable=False),
    sa.Column("parent_device_id", sa.Integer, sa.ForeignKey('device.device_id')),
    )

class Device(object):
    device_id = None
    def __init__(self, name, parent_device_id=None):
        self.name = name
        self.parent_device_id = parent_device_id

    def __repr__(self):
        return "<Device(%s, '%s', %s)>" % (self.device_id,
                                           self.name,
                                           self.parent_device_id )

mapper(Device, device_table)

metadata.create_all(engine)

db_session = sessionmaker(bind=engine)()

parent = Device('parent')
db_session.add(parent)
db_session.commit()

child = Device('child', parent.device_id)
db_session.add(child)
db_session.commit()

ParentDevice = aliased(Device, name='parent_device')
q = db_session.query(Device, ParentDevice)\
    .outerjoin(ParentDevice,
               Device.parent_device_id==ParentDevice.device_id)

print list(q)

这给了我这个错误:

ArgumentError:无法确定'device'和'parent_device'之间的连接; 表之间有多个外键约束关系.请明确指定此联接的"onclause".

但是我指onclause的是加入.我该怎么做？

Answer 1

因为query.[outer]join,你指定为连接列表(与...不同expression.[outer]join.)所以我需要将连接的2个元素,表和onclause元组放在一个元组中,如下所示:

q = db_session.query(Device, ParentDevice)\
    .outerjoin(
                  (ParentDevice, Device.parent_device_id==ParentDevice.device_id)
              )