find all that are nil in the association

Question

So I have a Post and a User.
Post has_many users and a user belongs_to a post.
I need a find that will find all the Posts that dont have any users like the following:

Answer 1

I know this is tagged as Rails 3, but if you are using Rails 4, I've been doing it like this.

Post.where.not(user_id: User.pluck(:id))