title: Find records with multiple foreign key values in Rails
body: <p>I've got two models <strong>User</strong> and <strong>Post</strong> where a 
  <code>:user</code> <code>has_many :posts</code></p>
  
  <p>How do I find a list of users who have more than <code>n</code> posts?</p>
  
  <p>I'm looking for a simple statement like <code>where</code> rather than something to do with using scopes.</p>