serializer.rb

class PostSerializer < ActiveModel::Serializer  
  attributes :id, :title

  belongs_to :user

  def self.lazy_user(post)
    BatchLoader.for(post.user_id).batch do |user_ids|
      User.where(id: user_ids)
    end
  end

  def author
    self.class.lazy_user(object)
  end
end