receipt.rb

class Receipt < ActiveRecord::Base
	private
	
	def calculate_total
		self.total = (self.sub_total || 0.0) + (self.tax_1 || 0.0) + (self.tax_2 || 0.0)
	end
end

receipt_spec.rb

require 'spec_helper'

describe Receipt do
	describe 'calculate_total' do
		class Receipt
			def test_calculate_total
				calculate_total
			end
		end
		
		it 'should add sub_total, tax_1, and tax_2, assigning the result to total' do
			receipt = Receipt.new({
				:payee_id => 1,
				:category_id => 1,
				:transacted_on => '2010-01-01',
				:sub_total => 1,
				:tax_1 => 1,
				:tax_2 => 1
			})
			receipt.test_calculate_total
			receipt.total.should == 3
		end
	end
end