严重依赖fixtures的测试会变得十分脆弱，并且很难维护。
让我们来看看怎样写不使用fixtures的测试。

看cart/line_item的例子:

class Cart < ActiveRecord::Base
  has_many :line_items

  def total_weight
    line_items.to_s.sum(&:weight)
  end
end

class LineItem < ActiveRecord::Base
  belongs_to :cart
  belongs_to :product
  belongs_to :delivery_method

  def weight
    if delivery_method.shipping?
      product.weight*quantity
    else
      0
    end
  end
end

对简单的字段数据我们可以通过使用ActiveRecord生成的build_xxx方法给字段赋值来测试:

class LineItemTest < Test::Unit::TestCase
  def test_should_have_zero_for_weight_when_not_shipping
    line_item = LineItem.new
    line_item.build_delivery_method(:shipping => false)
    assert_equal 0, line_item.weight
  end

  def test_should_have_weight_of_product_times_quantity_when_shipping
    line_item = LineItem.new(:quantity => 3)
    line_item.build_delivery_method(:shipping => true)
    line_item.build_product(:weight => 5)
    assert equal 15, line_item.weight
  end
end

但是我们考虑下面的测试:

class CartTest < Test::Unit::TestCase
  fixtures :carts, :line_items, :delivery_methods, :products

  def test_total_weight
    assert_equal 10, carts(:primary).total_weight
  end
end

这里我们手动准备cart/line_item数据会非常麻烦，而且total_weight依赖于line_item.weight方法，而weight方法已经测试过了
为了减少麻烦，我们可以使用mock方式来简化测试而不需要fixtures

我们需要先安装mocha

sudo gem install mocha

然后在test_helper.rb里require mocha:

require File.expand_path(File.dirname(__FILE__) + "/../config/environment")
require 'test_help'
require 'mocha'

我们这样使用mocha来写CartTest:

require File.dirname(__FILE__) + '/../test_helper'

class CartTest < Test::Unit::TestCase
  def test_total_weight_should_be_sum_of_line_item_weights
    cart = Cart.new
    cart.line_items.build.stubs(:weight).returns(7)
    cart.line_items.build.stubs(:weight).returns(3)
    assert_equal 10, cart.total_weight
  end
end

由于前面已经测过了weight方法，这里使用stubs模拟weight方法的返回值，将测试逻辑专注于total_weight方法