DRYing Up Preloads in Rails

As a Rails application grows, I’ve found that this pattern often emerges where we are repeating the same preloads across the application.

Post.preload(:user, :category, comments: { user: :avatar }).where(something: true).limit(100)

Whenever we add an association, or make a change. We have to update our preloads in multiple places. If we forget one, we’ve just introduced a potentially severe performance problem in our application.

(If you’re not familiar with includes/preloads/n+1’s, read this.)

We can DRY up this code by extracting it to a scope on the model.

We’ve been using this pattern in Product Hunt’s codebase for a few months now and I like it a lot.

scope :with_preloads, -> { preload preload_attributes }

class << self
  def preload_attributes
    [:user, :category, comments: { user: :avatar }]
  end
end

Now in our code, we can use our new scope like this.

# Old 😞
Post.preload(:user, :category, comments: { user: :avatar }).where(something: true).limit(100)

# New & Improved 😎
Post.with_preloads.where(something: true).limit(100)

Another benefit of adding this scope to all of our models is that it starts to look strange to see a query without with_preloads. It acts as a nice reminder to double check everything is being correctly loaded.