test.rb

module Purchasable
  def purchase( attrs = nil )
    Purchase.new( attrs || purchase_attributes ).process
  end

  class Purchase
    attr_reader :total, :card

    def initialize( options = {} )
      @total, @card = options[ :total ] || 0, options[ :card ] || test_card
    end

    def process
      transaction.success?
    end

    private

    def transaction
      @transaction ||= BrainTree::Transaction.sale( amount: total, credit_card: card )
    end

    def test_card
      '00000000000000'
    end
  end
end

class Order < ActiveRecord::Base
  include Purchasable
  belongs_to :user

  # mean to retrieve value can change any time
  def purchase_attributes
    { total: compute_total, card: user.credit_card }
  end
end