Adding related data to ActiveRecords without using a join - ruby-on-rails

Question

My Rails 4 app has a User model, a Link model, and a Hit model. Each User has many Links, and each Link has many Hits. Occasionally, I want to display a list of the User's Links with the number of Hits it has.
The obvious way to do this would be to loop over the links and call link.hits.count on each one, but this produces N+1 queries. So instead, I wrote a scope which joins the hits table:
scope :with_hit_counts, -> {
joins("LEFT OUTER JOIN hits ON hits.link_id = links.id").select('links.*', 'count(hits.link_id) AS hit_count').group("links.id")
}
This effectively adds a virtual hit_count attribute to each Link, which is computed in a single query. Curiously, it appears to be a separate query from loading the links, rather than actually being done in the same query:
SELECT COUNT(*) AS count_all, links.id AS links_id
FROM "links" LEFT OUTER JOIN hits ON hits.link_id = links.id
WHERE "links"."user_id" = $1
GROUP BY links.id
ORDER BY "links"."domain_id" ASC, "links"."custom_slug" ASC, "links"."id" ASC ;
Unfortunately, as the hits table grows, this has become a slow query. EXPLAIN indicates that the query is joining all hits with their matching links using an index, and then narrowing the links down to just the ones with the correct user_id by sequential scan; that seems to be the reason it's slow. However, if we're already loading the list of links separately—and we are—there's no actual need to join the links table at all. We can get the list of link IDs for the user and then do a query purely on the hits table with hits.link_id IN (list of IDs).
It's easy to write this as a separate query, and it runs lightning-fast:
Hit.where(link_id: #user.links.ids).group(:link_id).count
The problem is, I can't figure out how to get ActiveRecord to do this as a scope on the Link model, so that each Link has a hit_count attribute I can use, and so that I can use the resulting return value as a relation with the ability to chain other queries onto it. Any ideas?
(I do know about ActiveRecord's counter_cache feature, but I don't want to use it here—hits are inserted by a separate, non-Ruby system, and modifying that system to update the counter cache would be moderately painful.)

Answer 1

As trh mentioned, the common way to go about this is to add a hits_count to link.rb Then you can query and sort quickly. You just have to keep them in sync. If it truly is a basic count, then you can use the basic rails counter cache. This will increment and decrement on create and destroy.
class AddCounterCacheToLink < ActiveRecord::Migration
def up
add_column :links, :hits_count, :integer, :default => 0
Link.reset_column_information
Link.all.each do |l|
l.update_attribute :hits_count, l.hits.size
end
end
end
And then in the model
class Hit < ActiveRecord::Base
belongs_to :link, :counter_cache => true #this will trigger the rails magic
If you have something more complicated, you can write your own, which is trivial.