Sqlalchemy 具有额外字段的多对多映射

Question

我与 sqlalchemy 创建了一个多对多的关系，如下所示：

subject_books = Table('subject_books', Base.metadata,
    Column('subject_id', Integer, ForeignKey('subjects.id')),
    Column('book_id', Integer, ForeignKey('books.id')),
    Column('group', Integer)
)

class Subject(Base):
    __tablename__ = 'subjects'
    id = Column(Integer, primary_key=True)
    value = Column(Unicode(255), unique=True)

class Book(Base):
    __tablename__ = 'books'
    id =  Column(Integer, primary_key=True)
    title = Column(Unicode(255))
    isbn = Column(Unicode(24))
    subjects = relationship('Subject', secondary=subject_books, collection_class=attribute_mapped_collection('group'), backref='books')

之后我创建了一个如下所示的测试：

book = Book(title='first book',isbn='test')
book.subjects[0] = Subject(value='first subject')
book.subjects[1] = Subject(value='second subject')

session.add(book)
transaction.commit()

并且它工作正常。但我真正想要的是存储多个具有相同 group 值的主题，所以我尝试了以下测试：

book = Book(title='first book',isbn='test')
book.subjects[0] = [Subject(value='first subject'),Subject(value='second subject')]
book.subjects[1] = [Subject(value='third subject'),Subject(value='forth subject')]

session.add(book)
transaction.commit()

但它不起作用。

这可以使用 sqlalchemy 来完成吗？

提前致谢

Answer 1

拉齐

我认为你正在构建错误的关系。

您的关系必须是

book M2M subject

subject M2M group

因此，您必须为群组再创建一个模型，并且必须在中指定为 m2m主题

你的模型会是这样的。

subject_books = Table('subject_books', Base.metadata,
    Column('subject_id', Integer, ForeignKey('subjects.id')),
    Column('book_id', Integer, ForeignKey('books.id')),        
)
subject_group = Table('subject_groups', Base.metadata,
    Column('group_id', Integer, ForeignKey('groups.id')),
    Column('subject_id', Integer, ForeignKey('subjects.id')),
)

class Subject(Base):
    __tablename__ = 'subjects'
    id = Column(Integer, primary_key=True)
    value = Column(Unicode(255), unique=True)
    groups = relationship('Groups', secondary=subject_groups, backref='subjects')

class Groups(Base):
    __tablename__ = 'groups'
    id = Column(Integer, primary_key=True)
    name = Column(Unicode(255), unique=True)

class Book(Base):
    __tablename__ = 'books'
    id =  Column(Integer, primary_key=True)
    title = Column(Unicode(255))
    isbn = Column(Unicode(24))
    subjects = relationship('Subject', secondary=subject_books, backref='books')

我还检查了 attribute_mapped_collection 的文档。但每次我发现每个键只与一个对象相关联，而不是多个。如果您在任何地方阅读过，请提供链接，以便我可以检查它如何适合您的代码。

我想这会对你有所帮助。