ArLazyPreload

ArLazyPreload is a gem that brings association lazy load functionality to your Rails applications. There is a number of built-in methods to solve N+1 problem, but sometimes a list of associations to preload is not obvious–this is when you can get most of this gem.

• Simple. The only thing you need to change is to use

  #lazy_preload

  instead of

  #includes

  ,

  #eager_load

  or

  #preload

• Fast. Take a look at benchmarks (

  TASK=bench

  and

  TASK=memory

  )
• Perfect fit for GraphQL. Define a list of associations to load at the top-level resolver and let the gem do its job
• Auto-preload support. If you don't want to specify the association list–set

  ArLazyPreload.config.auto_preload

  to

  true

Why should I use it?

Lazy loading is super helpful when the list of associations to load is determined dynamically. For instance, in GraphQL this list comes from the API client, and you'll have to inspect the selection set to find out what associations are going to be used.

This gem uses a different approach: it won't load anything until the association is called for a first time. When it happens–it loads all the associated records for all records from the initial relation in a single query.

Usage

Let's try

#lazy_preload

users = User.lazy_preload(:posts).limit(10)  # => SELECT * FROM users LIMIT 10
users.map(&:first_name)

However, when we try to load posts, there will be one more request for posts:

users.map(&:posts) # => SELECT * FROM posts WHERE user_id in (...)

Auto preloading

If you want the gem to be even lazier–you can configure it to load all the associations lazily without specifying them explicitly. To do that you'll need to change the configuration in the following way:

ArLazyPreload.config.auto_preload = true

After that there is no need to call

#lazy_preload

If you want to turn automatic preload off for a specific record, you can call

.skip_preload

users.first.skip_preload.posts # => SELECT * FROM posts WHERE user_id = ?

Relation auto preloading

Another alternative for auto preloading is using relation

#preload_associations_lazily

posts = User.preload_associations_lazily.flat_map(&:posts)
# => SELECT * FROM users LIMIT 10
# => SELECT * FROM posts WHERE user_id in (...)

Gotchas

1. Lazy preloading does not work for ActiveRecord < 6 when

   .includes

   is called earlier:

  Post.includes(:user).preload_associations_lazily.each do |p|
    p.user.comments.load
  end

1. When

   #size

   is called on association (e.g.,

   User.lazy_preload(:posts).map { |u| u.posts.size }

   ), lazy preloading won't happen, because

   #size

   method performs

   SELECT COUNT()

   database request instead of loading the association when association haven't been loaded yet (here is the issue, and here is the explanation article about

   size

   ,

   length

   and

   count

   ).

Installation

Add this line to your application's Gemfile, and you're all set:

gem "ar_lazy_preload"

License

The gem is available as open source under the terms of the MIT License.