Rails Collection#exists?

I've been using Rails for almost a decade, and find I still don't know even close to all it has to offer. Today, I was browsing through an open source app, and stumbled across this code:

unless feed.posts.exists?(@post.id)
  feed.posts << @post
end

I'd never seen or used exists? on a collection before, so I looked it up. And of course, there it is, right in the Rails Guides Association Basics - methods added by has many!

Digging in a bit, what arguments can exists? take? Here's the reference.

Turns out there a lot of options!. Integer, string, array, hash, false, or no args.

Well that's kinda neat! So what does the no args do?

It seems to behave similar to any?, and return true if there are any members of the collection, false if it's empty.

TLDR: Read the Rails Guides. A decade in, you'll probably still learn something!