tab/app/models/transaction.rb

# == Schema Information
#
# Table name: transactions
#
#  id          :integer          not null, primary key
#  debtor_id   :integer          not null
#  creditor_id :integer          not null
#  issuer_id   :integer          not null
#  issuer_type :string           not null
#  amount      :integer          default(0), not null
#  message     :string
#  created_at  :datetime         not null
#  updated_at  :datetime         not null
#

class Transaction < ActiveRecord::Base
  belongs_to :debtor, class_name: 'User'
  belongs_to :creditor, class_name: 'User'
  belongs_to :issuer, polymorphic: true

  after_save    :recalculate_balances
  after_destroy :recalculate_balances

  validates :amount, numericality: { greater_than: 0 }
  validate :different_debtor_creditor

  private

  def recalculate_balances
    creditor.calculate_balance!
    debtor.calculate_balance!
  end

  def different_debtor_creditor
    if self.debtor == self.creditor
      self.errors.add :base, "Can't write money to yourself"
    end
  end
end