title: Django admin queryset filtering by foreign key backwards relation
body: <p>I have models A and B, where B has a FK to A.</p>
  
  <p>I use django 1.3 and I need two django admin filters:</p>
  
  <pre><code>1) a.b_set.exists() # (True/False)
  
  2) not a.b_set.filter(some_condition=False).exists() # (True/False)
  </code></pre>
  
  <p>How can I achieve that? Sadly, I couldn't find any solutions by googling.</p>