Rails - Query n random records from an association

Question

I have two classes. User and Review (User has many reviews).

I am trying to find then best method to extract two random reviews from the association of User.reviews

I can use user.reviews.order("RANDOM()").limit(n) but if I'm not mistaken this will be very heavy on the DB if I have users with a large number of reviews...

I am using Postgres as my database.

Thoughts?

I think it is.. Ok.. as DB native method will take care of it.. — Arup Rakshit, Jan 28 '15 at 20:04
possible duplicate of [quick random row selection in Postgres](http://stackoverflow.com/questions/5297396/quick-random-row-selection-in-postgres) — ptd, Jan 28 '15 at 20:44

score 2 · Answer 1 · answered Jan 29 '15 at 18:03

A database independent solution would be:

user.reviews.where(id: user.reviews.pluck(:id).sample(2))

I always prefer database independent solutions because otherwise it doesn't make sense to use ActiveRecord at all. Obviously this solution does two DB requests compared to your RANDOM() approach. Performance wise it is a tricky question because the database can cache these requests. It can not cache a RANDOM() request.

Rails - Query n random records from an association

1 Answers1