rails where() sql query on array

I'll explain this as best as possible. I have a query on user posts:

@selected_posts = Posts.where(:category => "Baseball")

I would like to write the following statement. Here it is in pseudo terms:

User.where(user has a post in @selected_posts)

Keep in mind that I have a many to many relationship setup so post.user is usable.

Any ideas?

/EDIT

@posts_matches = User.includes(@selected_posts).map{ |user|

      [user.company_name, user.posts.count, user.username]

    }.sort

Basically, I need the above to work so that it uses the users that HAVE posts in selected_posts and not EVERY user we have in our database.