User model with association by :parent_username

user.rb

class User < ApplicationRecord
  belongs_to :parent, primary_key: :username,
                      foreign_key: :parent_username,
                       class_name: 'User',
                         optional: true
  has_many :children, primary_key: :username,
                      foreign_key: :parent_username,
                       class_name: 'User'
end