new_contents: from nucleus.app import DB
  
  class Role(DB.Model):
  	id = DB.Column(DB.Integer, primary_key=True)
  	name = DB.Column(DB.String(50), unique=True)
  
  class UserRoles(DB.Model):
  	id = DB.Column(DB.Integer, primary_key=True)
  	user_id = DB.Column(DB.Integer, DB.ForeignKey('user.id', ondelete='CASCADE'))
  	role_id = DB.Column(DB.Integer, DB.ForeignKey('role.id', ondelete='CASCADE'))
subject: add roles schema