add balance entity

app/models/user.rb

@@ -25,6 +25,7 @@ class User < ApplicationRecord
     :recoverable, :rememberable, :validatable
 
   has_many :documents, class_name: "UserDocument", dependent: :destroy
+  has_many :balances, dependent: :restrict_with_error
 
   validates :first_name, :last_name, :email, presence: true
   validates :email, uniqueness: true