Pattern Wank

entity_controller.rb

class EntityController < ApplicationController
  respond_to :json

  def create
    entity    = Entity.new(post_params)
    validator = EntityValidator.new(entity)

    if validator.passes?
      collection = EntityRepository.add(entity)
      respond_with EntityPresenter.new(entity)
    else
      render :json => validator.errors.to_json, :status => :unprocessable_entity
    end
  end
end

Made rookie DDD mistake of conflating Repository Pattern with DDD. Informative exercise all the same.