title: Get all items of some other items
body: <p>I will give an example:</p>
  
  <pre><code>Books.find(THIS_FIND_MAY_GET_MORE_THAN_ONE_BOOK).pages.all
  </code></pre>
  
  <p>I am expecting a set of pages... Every page returned should be contained in at least one book. Otherwise it shouldn't be in the set. How can I get such a set?</p>
  
  <p>My intuitive idea was looping over the books and then add every time, but i guess there may be a faster way (databasewise), as I am not interested in the books. I don't want to introduce further database links as I think they may lead to a inconsistent database.</p>
  
  <p>Any suggestions? Thanks!</p>