how to append data to json in ruby/rails?

The to_json method takes an option object as parameter . So what you can do is make a method in your item class called as message and have it return the text that you want as its value .

class Item  < ActiveRecord::Base
 def message
  "it works"
 end
end

render :json => item.to_json(:methods => :message)

item = Item.find(params[:id])
item["message"] = "it works"
render :json => item.to_json