[英]How can I access extra columns in SQLAlchemy a many-to-many relationship with an association object (without an association proxy)?
[英]SQLAlchemy order_by many to many relationship through association proxy
我在SQLAlchemy的Flask應用程序中使用關聯對象設置了多對多的關系。 然后,我在類之間設置了關聯代理,以提供更直接的訪問,而不是通過關聯對象。
以下是設置的縮寫示例:
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
我不能為我的生活弄清楚如何讓group.members
返回的成員按其last_name
排序?
為了對group.members
進行排序,您必須在加載GroupMembership關聯對象時讓Persons可用於排序。 這可以通過連接來實現。
在當前配置中,訪問group.members
首先加載group.members
對象,填充group.group_memberships
關系,然后在關聯代理訪問GroupMembership.person
關系屬性時為每個Person激活SELECT。
相反,您希望在同一查詢中Person.last_name
和Persons,按Person.last_name
排序:
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
您需要定義order_by='Person.last_name'
標量關系屬性GroupMembership.person
代替backref Group.group_memberships
,這可能看起來像做順理成章的事情。 另一方面, order_by
“表示加載這些項時應該應用的順序”,因此在使用連接加載時是有意義的。 由於您將加入多對一引用並且外鍵不可為空,因此您可以使用內部聯接。
根據給定的定義:
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']
此解決方案的一個缺點是, person
關系總是急切地加載,並且在查詢GroupMemberships時應用了ORDER BY:
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]:
...
...除非明確使用其他加載策略:
In [16]: session.query(GroupMembership).options(lazyload('person')).all()
2018-04-05 21:10:52,404 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2018-04-05 21:10:52,405 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
FROM group_memberships
2018-04-05 21:10:52,405 INFO sqlalchemy.engine.base.Engine ()
如果您不時需要備用訂單,則必須恢復發出使用顯式預先加載的完整查詢並按以下順序排序:
In [42]: g = session.query(Group).\
...: filter_by(id=1).\
...: join(GroupMembership).\
...: join(Person).\
...: options(contains_eager('group_memberships')
...: .contains_eager('person')).\
...: order_by(Person.last_name.desc()).\
...: one()
...:
In [43]: [m.last_name for m in g.members]
Out[43]: ['30', '29', '28', '27', '26', '25', '24', '23', '22', '21']
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