Eager Loading Polymorphic Associations in Ruby on Rails

For when .includes is not enough.

Photo by Paul Smith on Unsplash

Basic Rails Associations

Rails’ ActiveRecord associations permits relating one model to another. Take the example below:

class Post < ApplicationRecord
  has_many :comments
end
class Comment < ApplicationRecord
  belongs_to :post
end

A Post has many Comments. This allows us to call Post.first.comments to get all the comments associated with a post! ActiveRecord internally generates our required SQL queries:

SELECT "posts".* FROM "posts" WHERE "posts".id = 1
SELECT "comments".* FROM "comments" WHERE "comments".post_id

N+1 Queries and .includes

However, imagine we are serializing this data to return as part of a controller for the /posts index. In that case, instead of getting the comments for just one post, we would get it for multiple posts: Post.all.each { |p| puts p.comments }. What happens to our SQL queries?

SELECT "posts".* FROM "posts"
SELECT "comments".* FROM "comments" WHERE "comments"."post_id" = 1
SELECT "comments".* FROM "comments" WHERE "comments"."post_id" = 2
...
SELECT "comments".* FROM "comments" WHERE "comments"."post_id" = N