Question

I’m trying to optimise some N+1 queries in active record for the first time. There are 3 to kill – 2 went very easily with a .includes call, but I can’t for the life of me figure out why the third is still calling a bunch of queries. Relevant code below – if anyone has any suggestions, I’d be really appreciative.

CONTROLLER:

@enquiries = Comment.includes(:children).faqs_for_project(@project)

MODEL;

def self.faqs_for_project(project)
  Comment.for_project_and_enquiries(project, project.enquiries).where(:published => true).order("created_at DESC")
end

(and the relevant scope)

scope :for_project_and_enquiries, lambda{|p, qs| where('(commentable_type = ? and commentable_id = ?) or (commentable_type = ? and commentable_id IN (?))', "Project", p.id, "Enquiry", qs.collect{|q| q.id})}

VIEW:

...
= render :partial => 'comments/comment', :collection => @enquries
...

(and that offending line in the partial)

...
= 'Read by ' + pluralize(comment.acknowledgers.count, 'lead')
...

Two SQL queries are called for each comment. The 2 queries are:

SQL (2.8ms)  SELECT COUNT(*) FROM "users" INNER JOIN "acknowledgements" ON "users".id = "acknowledgements".user_id WHERE (("acknowledgements".feedback_type = 'Comment') AND ("acknowledgements".feedback_id = 177621))
CACHE (0.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 1295 LIMIT 1

I would have thought appending (:user, :acknowledgements) into the controller’s .includes would have solved the problem, but it doesn’t seem to have any effect. If anyone has any suggestions on what I’m missing, I’d be really appreciative

Answer 1

I believe in your Comment table you want to add a :acknowledgers_count column as a counter cache

has_many :acknowledgers, ....., counter_cache: true

You will need to create a migration to add the :acknowledgers_count column to the comments table. Rails should take care of the rest.

You can learn more about the ActiveRecord::CounterCache api here.

The count method in comment.acknowledgers.count is overloaded in ActiveRecord to first check if a counter cache column exists, and if it does, it returns that directly from the model (in this case the Comment model) without having to touch the database again.

Finally, there was very recently a great Railscast about a gem call Bullet that can help you identify these query issues and guide you toward a solution. It covers both counter caches and N+1 queries.

As @ismaelga pointed out in a comment to this answer, it’s a generally better practice to call .size instead of .count on a relation. Check out the source for size:

def size
  loaded? ? @records.length : count
end

If the relation is already loaded it will just call length on it, otherwise it will call count. It’s an extra check to try and prevent the database from unnecessarily being queried.