Acts as API Example: I am using acts_as_api gem here to define this simple API this is part of overall much complex API, The Mongoid based model is part of tree structure for which we have used Mongoid-tree gem, This is why the model class inherits from Node class

categories_controller.rb

class CategoriesController < ApplicationController
  respond_to :html, :json
  before_filter :find, :only => [:edit, :update, :destroy]

  def index
    parent = load_node_from_params
    @nodes = parent.descendants.categories.sort
    respond_with @nodes.to_a, :api_template => :basic
  end

  def new
    @category = Category.new
    respond_with :@category
  end

  def create
    @category = Category.new(params[:category])
    @category.parent ||= @methodology
    @category.save
    respond_with @category
  end

  def update
    @category.update_attributes(params[:category])
    respond_with @category
  end

  def edit
    respond_with @category
  end

  def destroy
    @category.destroy
    respond_with @category
  end

  private

  def find
    @category = Category.where(:id => params[:id]).first
    redirect_to(root_path && return) unless @category
  end
end

category.rb

class Category < Node
  extend ActsAsApi::Base  
  
  acts_as_api

  api_accessible :detailed do |t|
    t.add :name
    t.add :uri
    t.add :category_uri
    t.add :ancestor_names
    t.add :question
  end

  api_accessible :basic do |t|
    t.add :name
    t.add :uri
    t.add :category_uri
    t.add :ancestor_names
    t.add :question
  end

  validates :parent, :type=>[Account, Category]
  validates :children, :type=>[Category, Template, ValueTable]

  delegate :account, :to => :parent

  def emission_factor_groups
    children.emission_factor_group
  end

  def uri
    generate_path
    path.split('/').drop(1).join('/')
  end

end