SQLAlchemy ManyToMany secondary table with additional fields

Question

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?

score 72 · Accepted Answer · edited Mar 10 '23 at 19:48

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")

I used your solution and it worked, but I still have a problem. Is it possible that Community.members (added with the association proxy) return an AppenderBaseQuery that I can add more filters, e.g. Community.members.filter_by(...)? I was used to do that with the plain many-to-many relationship using lazy=dynamic in the relationship declaration, but after giving a class mapping to the association table it seems that I cannot do it anymore. — stefanobaldo, Jan 30 '15 at 20:20
Did you ever discover the solution @stefanobaldo - would be really helpful if you did! — pip, Apr 18 '16 at 18:08

score 0 · Answer 2 · answered Jun 13 '17 at 13:01

If you only need query community_members and community table by a known user_id(such as user_id=2), In SQLAlchemy, you can perform:

session.query(community_members.c.time_create, Community.name).filter(community_members.c.user_id==2)

to get the result.

SQLAlchemy ManyToMany secondary table with additional fields

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