Last Updated: February 25, 2016
· mschae

Prefetch associations for existing model instances

Most of us will know and use this frequently:

Model.includes(:another_model, :yet_another_model, some_model_with: :associated_models).all

But what if (e.g. out of a cache or a complicated join query) you already have model instances and now would want to prefetch its corresponding associations?

There is an easy way to do this:

ActiveRecord::Associations::Preloader.new(objects, [:another_model, :yet_another_model, some_model_with: :associated_models]).run

This will "cache" the associated objects within the objects. Important here is that it will use one query per model type to prefetch all the objects.

So this:

objects.each{|o| o.another_model.do_something }

will not result in one query per iteration.

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