Given that you have used Rails for a medium sized project, or that you are zealous with your queries to the database, you’ve probably heard of activerecord’s includes method to eager load relations in order to prevent the N+1 query problem.

Don’t fret if you haven’t, I’ll break it down for you. Imagine you have a Post model which may have one or more Comment, and that in your view you do something of the sort (assuming @posts = Post.all):

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...
<% @posts.each do |post| %>
  <article>
    <p><%= post.body %></p>

    <footer>
      <% post.comments.each do |comment|
        <p><%= comment.body %></p>
      <% end %>
    </footer>
  </article>
<% end %>
...

In this snippet, a query for fetching the comments is being performed for each post, so it’s N for N equal to the number of posts plus 1 queries to list the posts. For a big number of posts you’ll swamp your database.

In order to prevent this you can do @posts = Post.includes(:comments).all that will eager load all the comments of all the posts and store them in memory, so when you try to access them you don’t hit the database.

Nested Eager Loading

A feature that I find rather undocumented is that it is possible to eager load a relation of an object you’re eager loading, such as the author of a comment if you wanted to access it’s name or avatar, for example.

That can be achieved by passing an Hash as an argument to the includes method instead of a Symbol, for the example above it would be, @posts = Post.includes(comments: :author).all.

I have tried it to three levels of nesting which is already more than you should ever need, still it can go deeper than that, not sure how much deeper, though.