title: Scala with keyword usage
body: <p>I found simple example:</p>
  
  <pre><code>class Post extends LongKeyedMapper[Post] with IdPK {
      def getSingleton = Post
  
      object title extends MappedText(this)
      object text extends MappedText(this)
      object date extends MappedDate(this)
  }
  
  
  object Post extends Post with LongKeyedMetaMapper[Post] {
      def getPosts(startAt: Int, count: Int) = {
          Post.findAll(OrderBy(Post.date, Descending), StartAt(startAt), MaxRows(count))
      }
  
      def getPostsCount = Post.count
  }
  </code></pre>
  
  <p>What does it mean <code>with IdPK</code> ? </p>
  
  <p>Thanks.</p>