Including associations optimization in Rails

Question

Hey,

I'm looking for help with Ruby optimization regarding loading of associations on demand.

This is simplified example. I have 3 models: Post, Comment, User. References are: Post has many comments and Comment has reference to User (:author). Now when I go to the post page, I expect to see post body + all comments (and their respective authors names). This requires following 2 queries:

select * from Post -- to get post data (1 row)
select * from Comment inner join User -- to get comment + usernames (N rows)

In the code I have:

Post.find(params[:id], :include => { :comments => [:author] }

But it doesn't work as expected: as I see in the back end, there're still N+1 hits (some of them are cached though). How can I optimize that?

UPD After some investigation, it looks like code was correct, but it doesn't work as expected in case I have named belongs_to in a Comment model. Once I changed from :author to :user, it worked as expected.

Answer 1

If you are happy with 2/3 queries, you can try:

@post = Post.find params[:id]
@comments = Comments.find_by_post_id(params[:id], :include => [:author])

or

@comments = @post.comments(:include => [:author])

Edit: Have you tried with:

Post.find(params[:id], :include => { :comments => :author }

Answer 2

In my project I have a similar relationship to your Post, Comment, and User models. I only see three actual sql queries.

Post.find(1, :include => { :comments => [:author] })

From the debug log it shows these three queries

SELECT * FROM `posts` WHERE (`posts`.`id` = 1)
SELECT `comments`.* FROM `comments` WHERE (`comments`.`post_id` = 1)
SELECT * FROM `authors` WHERE (`authors`.`id` IN (4,8,15,16,23,42))