item.js

$(document).ready(function(){
  $('a#save').click(function(event){
      event.preventDefault();
      $.ajax({
        type: "POST",
        url: '/items/' + id,
        data: $(this).siblings('input').serializeArray(),
        dataType: 'json'
      });
  });
});

Items.rb

class Items < ActiveRecord::Base
  def update
    @item = Items.find(params[:id])
    raise NotFound unless @item
    if @item.update_attributes(params[:item])
      redirect url(:item, item)
    else
      raise BadRequest
    end
  end
end