items_controller.rb

class ItemsController < ApplicationController
  include JsonParams

  def index
    @items = Item.all
    respond_to do |format|
      format.json { render :json => @items.to_json }
    end
  end

  def create
    @item = Item.new(json_params(params))
    @item.save!
    respond_to do |format|
      format.json { render :json => @item, :status => :created, :location => @item}
    end
  end

  def update
    @item = Item.find(params[:id])
    @item.update_attributes(json_params(params))
    respond_to do |format|
      format.json { render :json => @item}
    end
  end

  def show
    @item = Item.find(params[:id])
    respond_to do |format|
      format.json { render :json => @item.to_json }
    end
  end
end

json_params.rb

module JsonParams
  def json_params(params)
    params.reject{|k,v| ignored_fields.include?(k.to_s)}
  end

  private

    def ignored_fields
      %w(format action controller id)
    end
end