(This post originally appeared on Burnside Digital’s blog on July 12, 2013. Alas, Burnside Digital’s blog is no more, so I’ve reposted it here.)
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:
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
endHowever, 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
endAnd voila – if the content is cached, no eager loading takes place. If the cache isn’t populated or is invalid, we eager load.
