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?

Answer:

You'll 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")