我想检查项目的哪些字段被修改.我尝试过两次可能的尝试,那些不行.

第一次尝试！

def edit
  @item = Item.find(params[:id])
  @item_before_update = @item.dup
end

def update
  @item = Item.find(params[:id])
  # compare @item_before_update and @item here,but @item_before_update is NIL !!!
end

二次尝试！
我寻找将数据从视图传递到控制器的方式,我不能.
edit.html.erb

<% @item_before_update = @item.dup %> # I thought @item_before_update can be read in update method of item controller. But NO.
<% params[:item_before_update] = @item.dup %> # And I also thought params[:item_before_update] can be read in update mothod of item controller. But AGAIN NO

<% form_for(@item) do |f| %>
# omitted
<% end %>

请让我知道如何解决这个问题:(