nested-resources.rb

class PostSerializer < ActiveModel::Serializer  
  attributes :id, :title, :details, :author
  def author
    object.get_author_lazily do |author|
      # Serialize the author after it has been loaded.     
      ActiveModelSerializers::SerializableResource
                             .new(author)
                             .as_json[:user]
    end
  end
end

class Post
  def get_author_lazily
    # The current post object is added to the batch here,
    # which is eventually processed when the block executes.   
    BatchLoader.for(self).batch do |posts, batch_loader|
      author_ids = posts.pluck(:author_id)
      User.where(:_id.in => author_ids).each do |user|
        modified_user = block_given? ? yield(user) : user
        post = posts.detect { |p| p.author_id == user._id.to_s }  
        # 'Assign' the user object to the right post.
        batch_loader.call(post, modified_user)      
      end    
    end  
  end
end