Filter by count of children using Rails 3

Question

I'd like to select posts that have one or more comments using Rails 3 and a single query.

I'm trying something like this:

Post.includes(:comments).where('count(comments.id)>0')

However I get this error:

ActiveRecord::StatementInvalid: PGError: ERROR:  aggregates not allowed in WHERE clause

I've googled this and similar approaches, group by, etc with no luck. Any help will be appreciated.