我需要一些模型,例如:
- 作品-例如文学作品。
- 工作者-例如作曲家,翻译或类似的人对工作有贡献。
因此,需要一个'type'
字段来通过分工来区分工人。作为SQLAlchemy的文档,这种情况可以受益于关联对象,如下所示:
class Work(base):
id = Column(Integer, primary_key=True)
name = Column(String(50))
description = Column(Text)
class Worker(base):
id = Column(Integer, primary_key=True)
name = Column(String(50))
description = Column(Text)
class Assignment(base):
work_id = Column(Integer, Foreignkey('work.id'), primary_key=True)
worker_id = Column(Integer, Foreignkey('worker.id'), primary_key=True)
type = Column(SmallInteger, nullable=True)
然而,如何利用backref和的可选连接条件来立即建立关系,实现每个Work
对象都可以通过不同的属性来检索和修改相应的Worker
(s)。例如:work = session.query(Work).get(1)
work.name
>>> 'A Dream of The Red Mansions'
work.composers
>>> [<Worker('Xueqin Cao')>]
work.translators
>>> [<Worker('Xianyi Yang')>, <Worker('Naidie Dai')>]
亦然:
worker = session.query(Worker).get(1)
worker.name
>>> 'Xueqin Cao'
worker.composed
>>> [<Work('A Dream of The Red Mansions')>]
worker.translated
>>> []
直接添加secondaryjoin
而不指定secondary
似乎是不可行的,此外,SQLAlchemy的文档指出:
当使用关联对象模式时,建议不要将关联映射表用作其他地方关系()的次要参数,除非该关系()包含选项viewonly=True。否则,如果在相关属性和相关对象上检测到类似的状态,SQLAlchemy可能会尝试在同一表上发出冗余的INSERT和DELETE语句。
那么,有没有一种方法可以既优雅又容易地建立这些关系呢?
一般有三种方法。
一个是,做一个"香草"设置,在没有区分"类型"的情况下设置"工作"/"工人"-然后,使用relationship()
为"作曲家","组成","翻译器","翻译"通过使用"次要"到Assignment.__table__
以及自定义连接条件,以及viewonly=True
。所以你只能通过香草属性来写。这里的缺点是"普通"集合和"特定"集合之间没有立即同步。
另一个是,与"香草"设置相同,但只是使用普通的Python描述符在内存中给出"composer","compose","translator","translned"视图,即[obj.worker for obj in self.workers if obj.type == 'composer']
。这是最简单的方法。无论您在"香草"集合中放入什么,都会在"过滤"集合中显示出来,SQL很简单,并且有更少的SELECT语句(每个Worker/Work一个而不是每个Worker/Work N个)。
最后,最接近你所要求的方法,使用主连接和反向引用,但注意与关联对象,反向引用是在工作/分配和分配/工作之间,而不是直接在工作/工作之间。这种方法最终可能会使用更多的SQL来获得结果,但它是最完整的,并且还具有自动编写"类型"的漂亮特性。我们也使用了"单向回退",因为赋值并没有一个简单的向外关联的方法(有办法做到这一点,但它会很乏味)。使用Python函数自动创建关系可以减少样板文件,注意这里我使用的是字符串"type",如果您向系统添加更多参数,它可以是整数:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy
Base = declarative_base()
def _work_assignment(name):
assign_ = relationship("Assignment",
primaryjoin="and_(Assignment.work_id==Work.id, "
"Assignment.type=='%s')" % name,
back_populates="work", cascade="all, delete-orphan")
assoc = association_proxy("%s_assign" % name, "worker",
creator=lambda worker: Assignment(worker=worker, type=name))
return assoc, assign_
def _worker_assignment(name):
assign_ = relationship("Assignment",
primaryjoin="and_(Assignment.worker_id==Worker.id, "
"Assignment.type=='%s')" % name,
back_populates="worker", cascade="all, delete-orphan")
assoc = association_proxy("%s_assign" % name, "work",
creator=lambda work: Assignment(work=work, type=name))
return assoc, assign_
class Work(Base):
__tablename__ = 'work'
id = Column(Integer, primary_key=True)
name = Column(String(50))
description = Column(Text)
composers, composer_assign = _work_assignment("composer")
translators, translator_assign = _work_assignment("translator")
class Worker(Base):
__tablename__ = 'worker'
id = Column(Integer, primary_key=True)
name = Column(String(50))
description = Column(Text)
composed, composer_assign = _worker_assignment("composer")
translated, translator_assign = _worker_assignment("translator")
class Assignment(Base):
__tablename__ = 'assignment'
work_id = Column(Integer, ForeignKey('work.id'), primary_key=True)
worker_id = Column(Integer, ForeignKey('worker.id'), primary_key=True)
type = Column(String, nullable=False)
worker = relationship("Worker")
work = relationship("Work")
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
session = Session(e)
ww1, ww2, ww3 = Worker(name='Xueqin Cao'), Worker(name='Xianyi Yang'), Worker(name='Naidie Dai')
w1 = Work(name='A Dream of The Red Mansions')
w1.composers.append(ww1)
w1.translators.extend([ww2, ww3])
session.add(w1)
session.commit()
work = session.query(Work).get(1)
assert work.name == 'A Dream of The Red Mansions'
assert work.composers == [ww1]
assert work.translators == [ww2, ww3]
worker = session.query(Worker).get(ww1.id)
assert worker.name == 'Xueqin Cao'
assert worker.composed == [work]
assert worker.translated == []
worker.composed[:] = []
# either do this...
session.expire(work, ['composer_assign'])
# or this....basically need composer_assign to reload
# session.commit()
assert work.composers == []