N+1 Queries, Batch Loading and Active Model Serializers

Gemfile

# ...
# https://github.com/exAspArk/batch-loader
gem 'batch-loader'

init-code.rb

class PostsController < ApplicationController  
  def index    
    posts = Post.all     
     
    render json: posts  
  end
end

class Post
  belongs_to :author, class_name: 'User'
end

class PostSerializer < ActiveModel::Serializer  
  attributes :id, :title, :details
  belongs_to :author 
end

post.rb

class Post < ApplicationRecord
  # ...
  
  def get_author_lazily
    BatchLoader.for(self).batch do |posts, batch_loader|
      User.where(:_id.in => posts.pluck(:author_id)).each do |user|
        # Modify the user through a given block, say, for serialization.
        modified_user = block_given? ? yield(user) : user
        batch_loader.call(posts.detect { |p| p.author_id == user._id.to_s }, modified_user)
      end
    end
  end
  # ...
end

posts_controller.rb

class PostsController < ApplicationController
  def index
    # Can't do Post.includes(:author) beacuse the author (User object)
    # is stored in an entirely different database: a MongoDB instance.
    posts = Post.all

    render json: posts
  end
end

PostSerializer.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

user.rb

class User
  include Mongoid::Document
  include Mongoid::Timestamps
  # ...
end

user_serializer.rb

class UserSerializer < ActiveModel::Serializer
  # ....
end

@luccasmaso yes, include works only for ORM-defined database relations, not custom attributes, which is what user has become in our case.
We can pass custom parameters and access them with @instance_options to achieve our goal:

# PostSerializer.rb
attribute :user, if: -> { @instance_options.key?(:with_user) && @instance_options[:with_user] }
# ...

render json: posts, with_user: true

You can also stick with include, use @instance_options[:include] and check if user was specified. But I'd probably not do this because it sort of goes against what the include option is supposed to be for. Also, in a way our point was to avoid include since it forced the n+1 queries to run. Having said that, there's nothing essentially wrong with using @instance_options[:include].

As far as nested associations are concerned, you can pass them to the explicit call to AMS;

ActiveModelSerializers::SerializableResource.new(user, include: [some: :nested_stuff]).as_json[:user]

Sorry for replying late. Don't know why I didn't get an email!

Hey, I digged in this topic a bit and created a plugin for ActiveModelSerializers - https://github.com/Bajena/ams_lazy_relationships

It eliminates the problem that @lucasmaso mentioned in his comment :)