[英]How to extend the `getter`-functionality of SQLAlchemy's association_proxy?
[英]How can SQLAlchemy association_proxy be used bi-directionally?
我正在与Flask-SQLAlchemy一起使用Association Objects和association_proxy
创建多对多关系,但是我遇到了一个问题:当我使用association_proxy
,可以对其进行设置,以便在追加新实例时进行设置到关系的一侧,将创建关联对象,但是如果我尝试从关系的另一侧添加实例,则不会将关联对象的构造函数传递给正确的实例。
这是一个简化的示例,其中Club
有很多People
,而People
有很多Club
,并且关联对象是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
我使用的是association_proxy
因为不仅仅Club
和Person
之间的连接很重要; 如果需要,我还希望能够检索Membership
的joined_at
属性。 在大多数情况下,虽然,我只将查询并追加到.members
一的Club
或.clubs
一个的Person
。 在此示例中,我可以通过调用person_instance.clubs.append(club_instance)
创建一个新的Membership
,但是由于Membership
的构造函数只能接受一个参数,因此如果我想如何调用club_instance.members.append(person_instance)
向后工作? 我可能会遍历club_instance.members
似乎很club_instance.members
,但必须记住不要append()
。 我想念什么吗?
在此Google网上论坛讨论中找到了我的答案。 这是相关的部分:
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))
我直接在类定义中分配了association_proxy
而不是按照上述事实进行分配,而且效果也很好。
这样的SO回答也非常有帮助, 它描述了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'))
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