Question

我正在尝试创建一个db结构，其中我有许多类型的内容实体，其中一个是Comment，可以附加到任何其他内容实体。

请考虑以下事项：

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.

如何指定onclause？

Answer 1

尝试将Comment.__mapper_args__补充到：

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

P.S。我不明白你需要什么_idref关系？如果您想在Entity预期的地方使用评论，您可以按原样传递Comment个实例。

Answer 2

补充@ nailxx的答案：如果你有许多依赖表，重复所有样板是很乏味的。解决方案：将其全部移至元类。

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，作为读者的练习：您可以省略_identity=…语句并使用类名，如setattr(cls,'__tablename__',…)行。或者不覆盖现有的__tablename__或__mapper_args__属性。

请务必测试dct而不是cls是否存在：后者会在父类中找到属性，此时您显然不想要这些属性。

在SQLALchemy中创建具有多态性的自引用表

2 个答案: