python - flask-SQLAlchemy + postgres unique constraint error -

i trying achieve 2 points right here:

1.- sort users bestfriend name
2.- sort users number of friend

this wrote:

class monkey(db.model):      __tablename__ = 'monkey'     __table_args__ = (         db.uniqueconstraint('best_friend'),     )      id = db.column(db.integer, primary_key=true)     name = db.column(db.string(64), index=true, unique=true)     age = db.column(db.integer)     email = db.column(db.string(128), index=true, unique=true)     best_friend = db.column(db.string, db.foreignkey('monkey.name'))     password = db.column(db.string(128))     is_admin = db.column(db.boolean)     number_friends = db.column(db.integer)      def __init__(self, name, age, email, password, best_friend=none, is_admin=false):         self.name = name         self.age = age         self.email = email         self.password = generate_password_hash(password)         self.best_friend = best_friend         self.is_admin = is_admin      def number_friends(self):         number = friendship.query.filter_by(monkey=self.id).count()         return number      def get_best_friend(self):         bf = monkey.query.filter_by(id=self.best_friend).first()         return bf.name      def is_friend_of(self, friend):         friendship = friendship.query.filter_by(monkey=self.id, friend=friend).first()         if (friendship == none):             return false         else:             return true    class friendship(db.model):     __tablename__ = 'friendship'     __table_args__ = (         db.primarykeyconstraint('monkey', 'friend'),     )      monkey = db.column(db.integer, db.foreignkey('monkey.id'))     friend = db.column(db.integer, db.foreignkey('monkey.id'))      def __init__(self, monkey, friend):         self.monkey = monkey         self.friend = friend 

so, can see can number of friends monkey using method number_friends(), problem cant figure out how use number order query

what can do?

here tried

monkeys = models.monkey.query.order_by(models.monkey.number_friends()).paginate(page, 5, false) 

so far know doesnt because have make instantiation use method right? there way order query way want sqlalchemy using method number_friends() ?