title: Django models - how to filter number of ForeignKey objects
body: <p>I have a models <code>A</code> and <code>B</code>, that are like this:</p>
  
  <pre><code>class A(models.Model):
    title = models.CharField(max_length=20)
    (...)
  
  class B(models.Model):
    date = models.DateTimeField(auto_now_add=True)
    (...)
    a = models.ForeignKey(A)
  </code></pre>
  
  <p>Now I have some <code>A</code> and <code>B</code> objects, and I'd like to get a query that selects all <code>A</code> objects that have less then 2 <code>B</code> pointing at them.</p>
  
  <p>A is something like a pool thing, and users (the B) join pool. if there's only 1 or 0 joined, the pool shouldn't be displayed at all.</p>
  
  <p>Is it possible with such model design? Or should I modify that a bit?</p>