assert_difference(expressions, difference = 1, message = nil) {...}

Test numeric difference between the return value of an expression as a result of what is evaluated in the yielded block.

没看懂文档，谁能给细说一下啊。附上一范例。

test "should create post" do
  assert_difference('Post.count') do
    post :create, :post => { :title => 'Hi', :body => 'This is my first post.'}
  end
  assert_redirected_to post_path(assigns(:post))
  assert_equal 'Post was successfully created.', flash[:notice]
end

非常感谢。