Lazy loading associations for the ActiveRecord models. #includes, #eager_load and #preload are built-in methods to avoid N+1 problem, but sometimes when DB request is made we don't know what associations we are going to need later (for instance when your API allows client to define a list of loaded associations dynamically). The only possible solution for such cases is to load all the associations we might need, but it can be a huge overhead.

This gem allows to set up lazy preloading for associations - it won't load anything until association is called for a first time, but when it happens - it loads all the associated records for all records from the initial relation in a single query.

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

gem "ar_lazy_preload"

For example, if we define the following relation

users = User.lazy_preload(:posts).limit(10)

and use it in the following way

users.map(&:first_name)

there will be one query because we've never accessed posts:

SELECT * FROM users LIMIT 10

However, when we try to load posts

users.map(&:posts)

there will be one more request for posts:

SELECT * FROM posts WHERE user_id in (...)

If you want the gem to be even more lazy - you can configure it to load all the associations lazily without specifying them explicitly. In order 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 on the association, everything would be loaded lazily.

Worried about the performance? Take a look at benchmarks (TASK=bench and TASK=memory)

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