Question

我正在使用Flask-SQLAlchemy使用Association Objects和association_proxy创建多对多关系，但我遇到了一个问题：当我使用association_proxy时，我可以设置它，这样当我将一个新实例附加到关系的一侧时，将创建关联对象，但如果我尝试从关系的另一侧添加实例，则关联对象构造函数赢了传递正确的实例。

以下是一个简化示例，Club有很多People，People有很多Club，而关联对象是Membership：

class Club(db.Model):
    __tablename__ = 'clubs'

    id = db.Column(db.Integer, db.Sequence('clubs_id_seq'), autoincrement=True, primary_key=True)
    name = db.Column(db.String(255))
    memberships = db.relationship('Membership', backref='club')
    members = association_proxy('memberships', 'club') # Can I pass a Person instance to Membership here? 
    created_at = db.Column('created_at', db.DateTime, default=datetime.datetime.now())

    def __init__(self, name):
        self.name = name

class Person(db.Model):
    __tablename__ = 'people'

    id = db.Column(db.Integer, db.Sequence('people_id_seq'), autoincrement=True, primary_key=True)
    name = db.Column(db.String(255))
    memberships = db.relationship('Membership', backref='person')
    clubs = association_proxy('memberships', 'club')
    created_at = db.Column('created_at', db.DateTime, default=datetime.datetime.now())

    def __init__(self, name):
        self.name = name

class Membership(db.Model):
    __tablename__ =  'memberships'

    person_id = db.Column('person_id', db.Integer, db.ForeignKey('people.id'), primary_key=True)
    club_id = db.Column('club_id', db.Integer, db.ForeignKey('clubs.id'), primary_key=True)
    club = db.relationship('Club', backref='membership')
    joined_at = db.Column('joined_at', db.DateTime, default=datetime.datetime.now())

    def __init__(self, club):
        self.club = club

我正在使用association_proxy，因为Club和Person之间的联系不仅重要;我还希望能够在需要时检索joined_at的{{1}}属性。但是，在大多数情况下，我只会查询并附加Membership .members或Club .clubs Person。在此示例中，我可以通过调用Membership来创建新的person_instance.clubs.append(club_instance)，但由于Membership的构造函数只能使用一个参数，如果我可以调用club_instance.members.append(person_instance)是向后工作？我可能会迭代club_instance.members似乎很蹩脚，但必须记住不要append()。我错过了什么吗？

Answer 1

在this Google Groups discussion找到我的答案。这是相关部分：

class DossierTarife(Base):
    __tablename__ = 'tarifer_dossier'
    IdDossier = Column(Integer, ForeignKey('dossier.IdDossier'), primary_key=True)
    IdAt = Column(Integer, ForeignKey('article_tarife.IdAt'), primary_key=True)

    dossier = relationship(Dossier, backref=backref("tarifer_dossier", cascade="all, delete-orphan"))
    article_tarife = relationship(ArticleTarife, backref=backref("tarifer_dossier"))

class Dossier(Base):
    __tablename__ = 'dossier'
    IdDossier = Column('IdDossier', Integer, primary_key=True)
   ...

class ArticleTarife(Base):
    __tablename__ = 'article_tarife'
    IdAt = Column('IdAt', Integer, primary_key=True)
   ...

Dossier.LesTar = association_proxy("tarifer_dossier", "article_tarife", creator=lambda art:DossierTarife(article_tarife=art))
ArticleTarife.LesTar = association_proxy("tarifer_dossier", "dossier", creator=lambda dos:DossierTarife(dossier=dos))

我直接在课程定义中指定了我的association_proxy，而不是在上述事实之后进行，而且也很有效。

同样非常有用的是this SO answer描述如何正确删除关联对象行：

user = relationship(User, backref=backref('user_to_groups', cascade='all, delete-orphan'))
group = relationship(Group, backref=backref('group_to_user', cascade='all, delete-orphan'))

SQLAlchemy association_proxy如何双向使用？

1 个答案: