新手问题 使用 render partial 的 collection 来代替 each，是不是就无法加入 index 了？
<%= @lista = News.all
render :partial => "newsitem",
:layout => "list_news",
:collection => @lista
%>
是不是使用上面的情况来做循环，就用不到each_with_index的类似的效果了？
Rails also makes a counter variable available within a partial called by the collection, named after the member of the collection followed by _counter. For example, if you're rendering @products, within the partial you can refer to product_counter to tell you how many times the partial has been rendered. This does not work in conjunction with the as: :value option.
根据这里说的不可以使用 as，但是我把 as 去掉了，还是不行。