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

1.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"
}

user.rb

  def as_json(options = {})
    super only: [
      :id,
      :first_name,
      :last_name,
      :full_name,
      :city,
      :state
    ], methods: [
        :avatar_url
    ]
  end

  def avatar_url(size = 48)
    hash = Digest::MD5.hexdigest(self.email.downcase)
    default = ERB::Util.u "http://robohash.org/#{hash}.png?size=#{size}x#{size}&bgset=any"
    "https://secure.gravatar.com/avatar/#{hash}.png?s=#{size}&d=#{default}"
  end

user_controller.rb

def show
  @user = current_user
  respond_to do | format |
    format.html
    format.json { render json: @user }
end