An example of a JSON presenter. It uses the Representative gem for JSON rendering.

goals.rb

class Goal < ActiveRecord::Base
  belongs_to :user
  has_many :checkpoints

  # Fields from schema.rb
  #create_table "goals", :force => true do |t|
  #  t.string   "description"
  #  t.string   "period"
  #  t.integer  "user_id"
  #  t.integer  "position"
  #  t.datetime "created_at"
  #  t.datetime "updated_at"
  #end

end

goals_controller.rb

class Api::GoalsController < ApplicationController

  respond_to :json
  before_filter :authenticate_user!

  def index
    goals = GoalsPresenter.new(current_user.goals, TimeSpan::Fortnight.new(Date.today))
    respond_to do |format|
      format.json { render :json => goals.to_json }
    end
  end
end

goals_presenter.rb

require 'representative/json'

class GoalsPresenter < DelegateClass(Array)

  def initialize(goals, span)
    @goals = goals
    @span = span
    super(@goals)
  end

  def to_json
    json = Representative::Json.new do |r|
      r.list_of(:goals, @goals) do |goal|
        r.element :description
        r.element :position
        r.element :period
        r.element :created_at, goal.created_at
        r.element :id
        r.element :history, "history" do
          r.element :start_date, @span.start_date
          r.element :end_date, @span.end_date
          r.list_of(:checkpoints, goal.checkpoints.in_timespan(@span).in_date_order) do |checkpoint|
            r.element :date
            r.element :achieved
            r.element :description
            r.element :created_at, checkpoint.created_at
            r.element :id
          end
        end
      end
    end
    json.to_s
  end
end