title: How to influence realm .distinct results
body: <p>I have a list of books in my realm database and I am querying the database thus: </p>
  
  <pre><code>Realm realm = Realm.getDefaultInstance();
  RealmResults&lt;Book&gt; bookCategories = realm.where(Book.class).distinctAsync("category").sort("category");
  </code></pre>
  
  <p>This is working as expected but I have a problem with it. Some of the books in a some categories does not have a book cover.</p>
  
  <p>For example: there's a category named "Thrillers" and the category has 10 books; the above query returns the first item in the category regardless of whether it has a book cover or not. So I wanted it to ignore the first item if it doesn't have a book cover and continue skipping till it finds an item with a book cover. If however it gets to the last item and it still doesn't have a book cover, then it should pick it.</p>
  
  <p>Please is there any way I make this happen?</p>
  
  <p>Note:
  Books with covers has <code>blah-blah.png</code> as cover values while does that doesn't have covers, the value of covers returns <code>null</code> or empty string. </p>