在SQLALchemy中创建具有多态性的自引用表

Question

我正在尝试创建一个数据库结构,其中我有许多类型的内容实体,其中一个,一个注释,可以附加到任何其他实体.

考虑以下:

from datetime import datetime
from sqlalchemy import create_engine
from sqlalchemy import Column, ForeignKey
from sqlalchemy import Unicode, Integer, DateTime
from sqlalchemy.orm import relation, backref
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Entity(Base):
    __tablename__ = 'entities'
    id = Column(Integer, primary_key=True)
    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
    edited_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
    type = Column(Unicode(20), nullable=False)
    __mapper_args__ = {'polymorphic_on': type}

# <...insert some models based on Entity...>

class Comment(Entity):
    __tablename__ = 'comments'
    __mapper_args__ = {'polymorphic_identity': u'comment'}
    id = Column(None, ForeignKey('entities.id'), primary_key=True)
    _idref = relation(Entity, foreign_keys=id, primaryjoin=id == Entity.id)
    attached_to_id = Column(Integer, ForeignKey('entities.id'), nullable=False)
    #attached_to = relation(Entity, remote_side=[Entity.id])
    attached_to = relation(Entity, foreign_keys=attached_to_id,
                           primaryjoin=attached_to_id == Entity.id,
                           backref=backref('comments', cascade="all, delete-orphan"))

    text = Column(Unicode(255), nullable=False)

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

这似乎是正确的,除了SQLAlchemy不喜欢有两个外键指向同一个父.它说ArgumentError: Can't determine join between 'entities' and 'comments'; tables have more than one foreign key constraint relationship between them. Please specify the 'onclause' of this join explicitly.

我该如何指定onclause？

Answer 1

尽量补充你Comment.__mapper_args__的:

__mapper_args__ = {
    'polymorphic_identity': 'comment',
    'inherit_condition': (id == Entity.id),
}

PS我还不明白你需要什么_idref关系？如果你想在Entity预期的地方使用评论,你可以Comment按原样传递实例.