使用 SQLAlchemy 删除级联外键约束错误

Question

我稍微修改了官方example for a directed graph并添加了cascade='all, delete-orphan，如果在child中定义了关系，建议级联删除。不过，当我尝试删除父级时，我得到一个外键约束错误。

我希望能够删除一个节点对象，并且所有相关的边也应该被删除。最好使用passive_deletes=True，我也没有让它工作。

我修改的完整源代码，删除了 mysql 连接数据：

"""a directed graph example."""

from sqlalchemy import Column, Integer, ForeignKey, \
    create_engine
from sqlalchemy.orm import relationship, sessionmaker, backref
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class Node(Base):
    __tablename__ = 'node'

    node_id = Column(Integer, primary_key=True)

    def higher_neighbors(self):
        return [x.higher_node for x in self.lower_edges]

    def lower_neighbors(self):
        return [x.lower_node for x in self.higher_edges]


class Edge(Base):
    __tablename__ = 'edge'

    lower_id = Column(
        Integer,
        ForeignKey('node.node_id'),
        primary_key=True)

    higher_id = Column(
        Integer,
        ForeignKey('node.node_id'),
        primary_key=True)

    lower_node = relationship(
        Node,
        primaryjoin=lower_id == Node.node_id,
        backref=backref('lower_edges', cascade='all, delete-orphan'))

    higher_node = relationship(
        Node,
        primaryjoin=higher_id == Node.node_id,
        backref=backref('higher_edges', cascade='all, delete-orphan'))

    def __init__(self, n1, n2):
        self.lower_node = n1
        self.higher_node = n2

engine = create_engine('mysql+pymysql:', echo=True)
Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)

session = sessionmaker(engine)()

# create a directed graph like this:
#       n1 -> n2 -> n1
#                -> n5
#                -> n7
#          -> n3 -> n6

n1 = Node()
n2 = Node()
n3 = Node()
n4 = Node()
n5 = Node()
n6 = Node()
n7 = Node()

Edge(n1, n2)
Edge(n1, n3)
Edge(n2, n1)
Edge(n2, n5)
Edge(n2, n7)
Edge(n3, n6)

session.add_all([n1, n2, n3, n4, n5, n6, n7])
session.commit()

session = sessionmaker(engine)()
session.query(Node).filter(Node.node_id==1).delete()
session.commit()

Answer 1

删除是通过 session.query().delete() 完成的，它不适用于 SQLAlchemy 级联，需要在数据库中级联才能工作。