sqlalchemy：以多对多关系保持关联表中的列值

Question

假设我有一个项目表和一个任务表。一个项目可能有许多任务，一个任务可能分配给多个项目。我有一个关联表project_task，里面有项目和任务的映射关系，还有一个额外的rate列，用来记录项目具体的任务率。

表：项目

• 项目标识
• 姓名
• 说明

表：任务

• 任务标识
• 姓名
• 说明
• 利率

表：Project_Task

• 项目标识
• 任务标识
• 费率

如何在 Sqlalchemy 中映射这种关系？我的目标是 project.tasks 应该给我一个与项目关联的任务对象列表，其中 task.rate 设置为 project_task 表中记录的速率。

非常感谢！

Answer 1

在映射中使用表连接：

import sqlalchemy

from sqlalchemy import Column
from sqlalchemy import Integer
from sqlalchemy import DECIMAL
from sqlalchemy import Unicode
from sqlalchemy import Text
from sqlalchemy import ForeignKey

from sqlalchemy.sql import join

from sqlalchemy.orm import relation
from sqlalchemy.orm import column_property
from sqlalchemy.orm import create_session

from sqlalchemy.ext.declarative import declarative_base

engine = sqlalchemy.create_engine('sqlite:///stackoverflow_6144557.db', echo = True)

Base = declarative_base(bind=engine)

class ProjectTask(Base):
    __tablename__ = 'project_task'

    projectid = Column(Integer, ForeignKey('project.projectid'), primary_key = True)
    taskid = Column(Integer, ForeignKey('task.taskid'), primary_key = True)
    project_rate = Column('rate', DECIMAL(12, 4))

class Task(Base):
    __tablename__ = 'task'

    taskid = Column(Integer, primary_key = True)
    name = Column(Unicode(255))
    description = Column(Text)
    rate = Column(DECIMAL(12, 4))

class Project(Base):
    __tablename__ = 'project'

    projectid = Column(Integer, primary_key = True)
    name = Column(Unicode(255))
    description = Column(Text)
    tasks = relation("ExtendedProjectTask", backref = "project", lazy = 'joined')

class ExtendedProjectTask(Base):
    __table__ = join(ProjectTask.__table__, Task.__table__)

    projectid = column_property(ProjectTask.projectid)
    taskid = column_property(Task.taskid, ProjectTask.taskid)
    name = column_property(Task.name)
    description = column_property(Task.description)
    task_rate = column_property(Task.rate)
    project_rate = column_property(ProjectTask.project_rate)

    @property
    def rate(self):
        if self.project_rate is None:
            return self.task_rate
        else:
            return self.project_rate

if __name__ == '__main__':
    Base.metadata.create_all(engine)
    session = create_session(engine)
    for project in session.query(Project).all():
        print "\n%r, %r, %r" % (project.projectid, project.name, project.description)
        for task in project.tasks:
            print "\t%r, %r, %r, %r" % (task.taskid, task.name, task.description, task.rate)

Answer 2

这个怎么样？我正在将 ProjectTask 委托给 Task 以获取任务属性。我只设置了一个 get 属性，但您可以轻松地添加 set 和 delete（您可以很容易地想到一种通用的方法）。

import sqlalchemy

from sqlalchemy import Column
from sqlalchemy import Integer
from sqlalchemy import DECIMAL
from sqlalchemy import Unicode
from sqlalchemy import Text
from sqlalchemy import ForeignKey

from sqlalchemy.orm import relation
from sqlalchemy.orm import create_session

from sqlalchemy.ext.declarative import declarative_base

engine = sqlalchemy.create_engine('sqlite:///stackoverflow_6144557.db')
Base = declarative_base(bind=engine)

class ProjectTask(Base):
    __tablename__ = 'project_task'

    projectid = Column(Integer, ForeignKey('project.projectid'), primary_key=True)
    taskid = Column(Integer, ForeignKey('task.taskid'), primary_key=True)

    project_rate = Column('rate', DECIMAL(12, 4))
    task = relation("Task", backref="project_tasks", lazy="joined")

    @property
    def name(self):
        return self.task.name

    @property
    def description(self):
        return self.task.description

    @property
    def rate(self):
        if self.project_rate is None:
            return self.task.rate
        else:
            return self.project_rate

class Project(Base):
    __tablename__ = 'project'

    projectid = Column(Integer, primary_key=True)
    name = Column(Unicode(255))
    description = Column(Text)

    tasks = relation(ProjectTask, backref="project", lazy="joined")

class Task(Base):
    __tablename__ = 'task'

    taskid = Column(Integer, primary_key=True)
    name = Column(Unicode(255))
    description = Column(Text)
    rate = Column(DECIMAL(12, 4))


Base.metadata.create_all(engine)

if __name__ == '__main__':
    session = create_session(engine)

    for project in session.query(Project).all():
        print "\n%r, %r, %r" % (project.projectid, project.name, project.description)
        for task in project.tasks:
            print "\t%r, %r, %r, %r" % (task.taskid, task.name, task.description, task.rate)