Cache balance in user

app/models/transaction.rb

@@ -16,7 +16,16 @@ class Transaction < ActiveRecord::Base
   belongs_to :debtor, class_name: 'User'
   belongs_to :creditor, class_name: 'User'
 
+  after_save    :recalculate_balances
+  after_destroy :recalculate_balances
+
   def client
     Client.find_by name: origin
   end
+
+  private
+  def recalculate_balances
+    creditor.calculate_balance!
+    debtor.calculate_balance!
+  end
 end

app/models/user.rb

@@ -20,4 +20,10 @@ class User < ActiveRecord::Base
   def transactions
     Transaction.where("creditor_id = ? OR debtor_id = ?", id, id)
   end
+
+  def calculate_balance!
+    balance = incoming_transactions.sum(:amount) -
+                outgoing_transactions.sum(:amount)
+    self.update_attribute :balance, balance
+  end
 end