如何使用SQLAlchemy构建多对多关系:一个很好的例子

Question

我已经阅读了关于构建多对多关系的SQLAlchemy文档和教程,但是当关联表包含多于2个外键时,我无法弄清楚如何正确地完成它.

我有一个项目表,每个项目都有很多细节.许多项目的细节可以相同,因此项目和细节之间存在多对多关系

我有以下内容:

class Item(Base):
    __tablename__ = 'Item'
    id = Column(Integer, primary_key=True)
    name = Column(String(255))
    description = Column(Text)

class Detail(Base):
    __tablename__ = 'Detail'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    value = Column(String)

我的关联表是(它在代码中的其他2之前定义):

class ItemDetail(Base):
    __tablename__ = 'ItemDetail'
    id = Column(Integer, primary_key=True)
    itemId = Column(Integer, ForeignKey('Item.id'))
    detailId = Column(Integer, ForeignKey('Detail.id'))
    endDate = Column(Date)

在文档中,据说我需要使用"关联对象".我无法弄清楚如何正确使用它,因为它与mapper表单混合声明,并且示例似乎不完整.我添加了这一行:

details = relation(ItemDetail)

作为Item类和行的成员:

itemDetail = relation('Detail')

作为关联表的成员,如文档中所述.

当我执行item = session.query(Item).first()时,item.details不是Detail对象的列表,而是ItemDetail对象的列表.

如何在Item对象中正确获取详细信息,即item.details应该是Detail对象的列表？

Answer 1

与 Miguel 一样，我也对连接表使用声明式方法。但是，我不断遇到类似的错误

sqlalchemy.exc.ArgumentError：辅助参数<class ' main .ProjectUser'>传递给relationship() User.projects必须是Table对象或其他FROM子句；无法直接发送映射的类，因为“辅助”中的行独立于映射到同一表的类而持久保存。

经过一番摆弄，我得出了以下结论。（请注意，我的课程与 OP 的课程不同，但概念是相同的。）

例子

这是一个完整的工作示例

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import declarative_base, relationship, Session

# Make the engine
engine = create_engine("sqlite+pysqlite:///:memory:", future=True, echo=False)

# Make the DeclarativeMeta
Base = declarative_base()


class User(Base):
    __tablename__ = "users"

    id = Column(Integer, primary_key=True)
    name = Column(String)
    projects = relationship('Project', secondary='project_users', back_populates='users')


class Project(Base):
    __tablename__ = "projects"

    id = Column(Integer, primary_key=True)
    name = Column(String)
    users = relationship('User', secondary='project_users', back_populates='projects')


class ProjectUser(Base):
    __tablename__ = "project_users"

    id = Column(Integer, primary_key=True)
    notes = Column(String, nullable=True)
    user_id = Column(Integer, ForeignKey('users.id'))
    project_id = Column(Integer, ForeignKey('projects.id'))



# Create the tables in the database
Base.metadata.create_all(engine)

# Test it
with Session(bind=engine) as session:

    # add users
    usr1 = User(name="bob")
    session.add(usr1)

    usr2 = User(name="alice")
    session.add(usr2)

    session.commit()

    # add projects
    prj1 = Project(name="Project 1")
    session.add(prj1)

    prj2 = Project(name="Project 2")
    session.add(prj2)

    session.commit()

    # map users to projects
    prj1.users = [usr1, usr2]
    prj2.users = [usr2]

    session.commit()


with Session(bind=engine) as session:

    print(session.query(User).where(User.id == 1).one().projects)
    print(session.query(Project).where(Project.id == 1).one().users)

笔记

1. secondary在参数中引用表名，secondary='project_users'而不是secondary=ProjectUser
2. 使用back_populates而不是backref

我在这里对此做了详细的文章。

Answer 2

从评论中我看到你找到了答案.但SQLAlchemy文档对于"新用户"而言非常压倒性,而我正在努力解决同样的问题.所以供将来参考:

ItemDetail = Table('ItemDetail',
    Column('id', Integer, primary_key=True),
    Column('itemId', Integer, ForeignKey('Item.id')),
    Column('detailId', Integer, ForeignKey('Detail.id')),
    Column('endDate', Date))

class Item(Base):
    __tablename__ = 'Item'
    id = Column(Integer, primary_key=True)
    name = Column(String(255))
    description = Column(Text)
    details = relationship('Detail', secondary=ItemDetail, backref='Item')

class Detail(Base):
    __tablename__ = 'Detail'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    value = Column(String)
    items = relationship('Item', secondary=ItemDetail, backref='Detail')

Answer 3

上一个答案对我有用，但我对表 ItemDetail 使用了类基方法。这是示例代码：

class ItemDetail(Base):
    __tablename__ = 'ItemDetail'
    id = Column(Integer, primary_key=True, index=True)
    itemId = Column(Integer, ForeignKey('Item.id'))
    detailId = Column(Integer, ForeignKey('Detail.id'))
    endDate = Column(Date)

class Item(Base):
    __tablename__ = 'Item'
    id = Column(Integer, primary_key=True)
    name = Column(String(255))
    description = Column(Text)
    details = relationship('Detail', secondary=ItemDetail.__table__, backref='Item')

class Detail(Base):
    __tablename__ = 'Detail'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    value = Column(String)
    items = relationship('Item', secondary=ItemDetail.__table__, backref='Detail')