post_representer.rb

module PostRepresenter
  include Representer
  using Representer
  
  def basic(post)
    select(post, :id, :name)
  end
  
  def details(post)
    basic(post) & comments(post)
  end
  
  def comments(post)
    { comments: post.comments.map {|c| CommentRepresenter.basic(c) }
  end
end

representer.rb

# requires ruby 2.1+
module Representer
  extend ActiveSupport::Concern

  refine Hash do
    alias_method :&, :merge
  end

  included do
    extend self
  end

  protected

  def select(obj, *fields)
    fields.map {|field| { field => obj.send(field) } }.inject({}, :merge)
  end
end