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，这个结构里有很多种内容实体，其中一种是评论（Comment），它可以附加到任何其他实体上。

考虑以下内容：

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)

这看起来差不多，但SQLAlchemy不喜欢有两个外键指向同一个父级。它提示说 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 呢？

Answer 1

为了补充@nailxx的回答：如果你有很多相关的表，重复写那些冗长的代码会很麻烦。解决办法是把这些代码放到一个元类里。

class EntityMeta(type(Entity)):
    def __init__(cls, name, bases, dct):
        ident = dct.get('_identity',None)
        if '__abstract__' not in dct:
            xid = Column(None, ForeignKey(Entity.id), primary_key=True)
            setattr(cls,'id',xid)
            setattr(cls,'__mapper_args__', { 'polymorphic_identity': dct['_identity'], 'inherit_condition': (xid == Entity.id,), 'primary_key':(xid,) })
            setattr(cls,'__tablename__',name.lower())
        super(EntityMeta, cls).__init__(name, bases, dct)

class EntityRef(Entity):
    __abstract__ = True
    __metaclass__ = EntityMeta

class Comment(EntityRef):
    _identity = 'comment'
    [...]

变体，留给读者练习：你可以省略_identity=…这一行，直接用类名，比如在setattr(cls,'__tablename__',…)这一行中。或者不覆盖已有的__tablename__或__mapper_args__属性。

确保检查dct是否存在，而不是cls：后者会查找父类中的属性，这在这个时候显然是不想要的。

Answer 2

试着把你的 Comment.__mapper_args__ 补充成：

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

顺便说一下，我不太明白你为什么需要 _idref 这种关系？如果你想在需要 Entity 的地方使用评论，你可以直接传递 Comment 实例就可以了。