SQLAlchemy order_by通过关联代理实现多对多关系

我在使用关联对象的SQLAlchemy中的Flask应用程序中设置了多对多关系。然后我在这些类之间建立了关联代理，以提供更直接的访问，而不是通过关联对象。SQLAlchemy order_by通过关联代理实现多对多关系

下面是设置的缩写例如：

class Person(Model): 
    __tablename__ = 'persons' 
    id = Column(Integer, primary_key=True) 
    last_name = Column(Text, nullable=False) 
    groups = association_proxy('group_memberships', 'group') 
    # Other stuff 

class Group(Model): 
    __tablename__ = 'groups' 
    id = Column(Integer, primary_key=True) 
    name = Column(Text, nullable=False) 
    members = association_proxy('group_memberships', 'person') 
    # Other stuff 

class GroupMembership(Model): 
    __tablename__ = 'group_memberships' 
    id = Column(Integer, primary_key=True) 
    person_id = Column(Integer, ForeignKey('persons.id'), nullable=False) 
    group_id = Column(Integer, ForeignKey('groups.id'), nullable=False) 
    person = relationship('Person', uselist=False, backref=backref('group_memberships', cascade='all, delete-orphan')) 
    group = relationship('Group', uselist=False, backref=backref('group_memberships', cascade='all, delete-orphan'))  
    # Other stuff

我不能为我的生活弄清楚什么是如何获得通过会员返回group.members由他们last_name进行排序？

来源

2017-06-20 Mr Alpha

你不在引用端定义关系时，不需要'uselist = False'参数，换句话说'GroupMembership'实体只能引用一个'Person'和'Group'。你会在一对一的关系中使用它。 –

可以肯定的是，你是否希望一般手动处理GroupMembership对象？换句话说，选择/查询GroupMembership是否很常见？ –

为了排序group.members您必须让人员在加载GroupMembership关联对象时进行排序。这可以通过连接来实现。

在当前的配置访问group.members首先加载GroupMembership对象，填充group.group_memberships关系，然后触发一个SELECT每个人作为关联代理访问GroupMembership.person关系属性。

而是要在相同的查询加载两个GroupMemberships和人员，通过Person.last_name分类：

class GroupMembership(Model): 
    __tablename__ = 'group_memberships' 
    id = Column(Integer, primary_key=True) 
    person_id = Column(Integer, ForeignKey('persons.id'), nullable=False) 
    group_id = Column(Integer, ForeignKey('groups.id'), nullable=False) 
    person = relationship('Person', 
          backref=backref('group_memberships', 
              cascade='all, delete-orphan'), 
          lazy='joined', innerjoin=True, 
          order_by='Person.last_name') 
    group = relationship('Group', backref=backref('group_memberships', 
                cascade='all, delete-orphan')) 
    # Other stuff

您需要在标量关系定义order_by='Person.last_name'属性GroupMembership.person代替backref Group.group_memberships，这可能似乎是合乎逻辑的事情。另一方面，order_by“表示加载这些项目时应该应用的顺序”，因此在使用joined loading时有意义。由于您将加入多对一的引用，并且外键不可为空，因此可以使用内部联接。

随着地方给出的定义：

In [5]: g = Group(name='The Group') 

In [6]: session.add_all([GroupMembership(person=Person(last_name=str(i)), group=g) 
    ...:     for i in range(30, 20, -1)]) 

In [7]: session.commit() 

In [8]: g.members 
2017-06-29 09:17:37,652 INFO sqlalchemy.engine.base.Engine BEGIN (implicit) 
2017-06-29 09:17:37,653 INFO sqlalchemy.engine.base.Engine SELECT groups.id AS groups_id, groups.name AS groups_name 
FROM groups 
WHERE groups.id = ? 
2017-06-29 09:17:37,653 INFO sqlalchemy.engine.base.Engine (1,) 
2017-06-29 09:17:37,655 INFO sqlalchemy.engine.base.Engine SELECT group_memberships.id AS group_memberships_id, group_memberships.person_id AS group_memberships_person_id, group_memberships.group_id AS group_memberships_group_id, persons_1.id AS persons_1_id, persons_1.last_name AS persons_1_last_name 
FROM group_memberships JOIN persons AS persons_1 ON persons_1.id = group_memberships.person_id 
WHERE ? = group_memberships.group_id ORDER BY persons_1.last_name 
2017-06-29 09:17:37,655 INFO sqlalchemy.engine.base.Engine (1,) 
Out[8]: [<__main__.Person object at 0x7f8f014bdac8>, <__main__.Person object at 0x7f8f014bdba8>, <__main__.Person object at 0x7f8f014bdc88>, <__main__.Person object at 0x7f8f01ddc390>, <__main__.Person object at 0x7f8f01ddc048>, <__main__.Person object at 0x7f8f014bdd30>, <__main__.Person object at 0x7f8f014bde10>, <__main__.Person object at 0x7f8f014bdef0>, <__main__.Person object at 0x7f8f014bdfd0>, <__main__.Person object at 0x7f8f0143b0f0>] 

In [9]: [p.last_name for p in _] 
Out[9]: ['21', '22', '23', '24', '25', '26', '27', '28', '29', '30']

这一解决方案的缺点是person关系总是急切地加载和ORDER BY查询GroupMemberships当应用：

In [11]: session.query(GroupMembership).all() 
2017-06-29 12:33:28,578 INFO sqlalchemy.engine.base.Engine BEGIN (implicit) 
2017-06-29 12:33:28,578 INFO sqlalchemy.engine.base.Engine SELECT group_memberships.id AS group_memberships_id, group_memberships.person_id AS group_memberships_person_id, group_memberships.group_id AS group_memberships_group_id, persons_1.id AS persons_1_id, persons_1.last_name AS persons_1_last_name 
FROM group_memberships JOIN persons AS persons_1 ON persons_1.id = group_memberships.person_id ORDER BY persons_1.last_name 
2017-06-29 12:33:28,578 INFO sqlalchemy.engine.base.Engine() 
Out[11]: 
    ...

来源

2017-06-29 06:18:32

SQLAlchemy order_by通过关联代理实现多对多关系

回答

相关问题