Sqlalchemy领域模型的设计问题？

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>]

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说明： 以上文字和代码来自so网站，看着合适，拿过来用一下来说明我的问题。
http://stackoverflow.com/questions/24477806/remove-a-relation-many-to-many-association-object-on-sqlalchemy

1.有3个表模型：User, Community, community_members
2.Community------------------User （many-----中间表--------many）

问题： 如何抽象一下代码？把代码放到domain service 里面