SQLAlchemy ManyToMany secondary table with additional fields

问题

I have 3 tables: User, Community, community_members (for relationship many2many of users and community).

I create this tables using Flask-SQLAlchemy:

community_members = db.Table(\'community_members\',
                db.Column(\'user_id\', db.Integer, db.ForeignKey(\'user.id\')),
                db.Column(\'community_id\', db.Integer, db.ForeignKey(\'community.id\')),
                )


class Community(db.Model):
    __tablename__ = \'community\'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), nullable=False, unique=True)
    members = db.relationship(User, secondary=community_members,
                            backref=db.backref(\'community_members\', lazy=\'dynamic\'))

Now I want add additional field to community_members like this:

community_members = db.Table(\'community_members\',
                db.Column(\'id\', db.Integer, primary_key=True),
                db.Column(\'user_id\', db.Integer, db.ForeignKey(\'user.id\')),
                db.Column(\'community_id\', db.Integer, db.ForeignKey(\'community.id\')),
                db.Column(\'time_create\', db.DateTime, nullable=False, default=func.now()),
                )

And now in python shell I can do this:

create community:

> c = Community()
> c.name = \'c1\'
> db.session.add(c)
> db.session.commit()

add members to community:

> u1 = User.query.get(1)
> u2 = User.query.get(2)
> c.members.append(u1)
> c.members.append(u2)
> db.session.commit()

> c.members
[<User 1>, <User 2>]

Ok, this works.

But how now I can get time_create of community_members table?

回答1:

You will have to switch from using a plain, many-to-many relationship to using an "Association Object", which is basically just taking the association table and giving it a proper class mapping. You'll then define one-to-many relationships to User and Community:

class Membership(db.Model):
    __tablename__ = 'community_members'

    id = db.Column('id', db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    community_id = db.Column(db.Integer, db.ForeignKey('community.id'))
    time_create = db.Column(db.DateTime, nullable=False, default=func.now())

    community = db.relationship(Community, backref="memberships")
    user = db.relationship(User, backref="memberships")


class Community(db.Model):
    __tablename__ = 'community'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), nullable=False, unique=True)

But you may only occasionally be interested in the create time; you want the old relationship back! well, you don't want to set up the relationship twice; because sqlalchemy will think that you somehow want two associations; which must mean something different! You can do this by adding in an association proxy.

from sqlalchemy.ext.associationproxy import association_proxy

Community.members = association_proxy("memberships", "user")
User.communities = association_proxy("memberships", "community")