class User(UserMixin, SurrogatePK, Model):
    __tablename__ = 'users'
    id = Column(db.Integer, primary_key=True, index=True)
    username = Column(db.String(80), unique=True, nullable=False)
    email = Column(db.String(80), unique=False, nullable=False)
    password = Column(db.String(128), nullable=True)
    departments = relationship("Department",secondary="user_department_relationship_table", back_populates="users")

user_department_relationship_table=db.Table('user_department_relationship_table',
                             db.Column('department_id', db.Integer,db.ForeignKey('departments.id'), nullable=False),
                             db.Column('user_id',db.Integer,db.ForeignKey('users.id'),nullable=False),
                             db.PrimaryKeyConstraint('department_id', 'user_id') )


class Department(SurrogatePK, Model):
    __tablename__ = 'departments'
    id = Column(db.Integer, primary_key=True, index=True)
    name = Column(db.String(80), unique=True, nullable=False)
    short_name = Column(db.String(80), unique=True, nullable=False)
    invite_codes = relationship("Invite_Code", backref="department")
    users = relationship("User", secondary=user_department_relationship_table,back_populates="departments")

from cpt import (
        public, user, department
        )

...


def register_blueprints(app):
    app.register_blueprint(public.views.blueprint)
    app.register_blueprint(user.views.blueprint)
    app.register_blueprint(department.views.blueprint)
    return None