http://blog.tutorwith.me/2011/09/how-to-customize-json-output/

search.json

[
  {
    city: "Montreal",
    first_name: "Istvan",
    id: 1,
    last_name: "Pusztai",
    state: "Quebec",
    avatar_url: "https://secure.gravatar.com/avatar/44c6f8371be4ee1612256855a3e35d12.png?s=48&d=http%3A%2F%2Frobohash.org%2F44c6f8371be4ee1612256855a3e35d12.png%3Fsize%3D48x48%26bgset%3Dany"
  },
  {
    city: "Laval",
    first_name: "Harry",
    id: 3,
    last_name: "Kavallos",
    state: "Quebec",
    avatar_url: "https://secure.gravatar.com/avatar/526bb7ac8a017f8f10a168c0afef7340.png?s=48&d=http%3A%2F%2Frobohash.org%2F526bb7ac8a017f8f10a168c0afef7340.png%3Fsize%3D48x48%26bgset%3Dany"
  }
]

user_controller.rb

  def search
    respond_to do | format |
      format.json { render json: User.search(params[:q]).all }
    end
  end