UsamaAshraf

class PostSerializer < ActiveModel::Serializer  
  attributes :id, :title, :details, :author
  # Will run n Mongo queries for n posts being rendered.
  def author
    User.find(object.author_id)
  end
end

# This is now a Mongoid document, not an ActiveRecord model.
class User  

UsamaAshraf

posts      = Post.all
author_ids = posts.pluck(:author_id)
authors    = User.where(:_id.in => author_ids)
# Somehow pass the author objects to the post serializer and
# map them to the correct post objects. Can't imagine what 
# exactly that would look like, but probably not pretty.
render json: posts, pass_some_parameter_maybe: authors

UsamaAshraf

class PostSerializer < ActiveModel::Serializer  
  attributes :id, :title, :details, :author
  def author
    object.get_author_lazily
  end
end

class Post
  def get_author_lazily
    # The current post object is added to the batch here,

UsamaAshraf

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