Conditionally Eager-load associations in Rails

This article was published 1 year, 5 months ago. Due to the rapidly evolving nature of web development, some concepts may no longer be applicable.

We recently had a project for a client where we wanted to eagerly load a model’s associations, but only under certain conditions – the associated models were rendered from a page fragment that was cached. It turns out this is pretty easy to do.

The project that we needed this solution for has a basic CMS. The schema for the CMS looks something like this:

cms-schema

In other words, our queries were fraught with opportunities for gobs and gobs of dreaded “N+1 queries.” To avoid the n+1 queries, we could simply eagerly load by invoking include:

class HomeController < ApplicationController
  def index
    @homepage = HomePage.find(params[:id], :include => 
      {:page_section =>
      {:page_section_schedule =>
      {:page_fragment =>
      {:carousel_slide => :image}}}})
  end
end

However, there was caching to consider – this data rarely changes (at most, daily), so it’s also a great candidate for fragment caching. With a fragment cache in place, the page looks a bit like this:

<html>
 <head>
  <title>Home Page</title>
 </head>
 <body>
  <h1>Home Page</h1>
  <div>
    <%- cache( "homepage-fragment-#{@homepage.cache_key}" ) do %>
      <%= render "content", :locals => { :homepage => @homepage } %>
    <%- end %>
  </div>
 </body>
</html>

With fragment caching, all that laborious eager loading isn’t necessary – the associations aren’t needed because the data is already cached in a fragment.

To get around this, we simply perform the eager loading only when we need it, in the cached fragment:

<html>
 <head>
  <title>Home Page</title>
 </head>
 <body>
  <h1>Home Page</h1>
  <div>
    <%- cache( "homepage-fragment-#{@homepage.updated_at}" ) do %>
      @homepage.do_eager_loading
      <%= render "content", :locals => { :homepage => @homepage } %>
    <%- end %>
  </div>
 </body>
</html>

The do_eager_loading method uses Rails’ internal eager loading methods to get the work done. It looks like this:

class HomePage < ActiveRecord::Base
  has_many :page_section_schedules

  def do_eager_loading
    ActiveRecord::Associations::Preloader.new([self], 
      {:page_sections => 
      {:page_section_schedules => 
      {:page_fragment => 
      {:carousel_slides => :image} }}}).run
  end
end

And voila – if the content is cached, no eager loading takes place. If the cache isn’t populated or is invalid, we eager load.

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