Ruby Quicktips

How to check if objects or relations exist

Here’s an interesting fact when checking if objects or relations exist in a collection.
To check if there were any items present in a collection you can do something like this:

Object.relation.present?

This, however, is better:

Object.relation.any?

Turns out that - when you request associated objects for the first time - the any? method will perform a COUNT (*) SQL query where as the present? method will perform a SELECT (*) which is infinitely slower than performing a count.

blog = Blog.first
blog.posts.present?
# SQL (284.1ms)   SELECT * FROM "posts" WHERE ("posts".blog_id = 1)

blog = Blog.first
blog.posts.any?
# SQL (1.2ms)   SELECT count(*) AS count_all FROM "posts" WHERE ("posts".blog_id = 1)

Thanks to Mario and Keith for this tip.