title: Django: query all items that have a foreign key point to them
body: <p>I have two models:</p>
  
  <pre><code>class A(models.Model):
      name = models.CharField(max_length=100, unique=True)
  
  class B(models.Model):
      a = models.ForeignKey(A)
  </code></pre>
  
  <p>In Django, how can I select all objects of class 'A' that have an object of class B that points to them? For example, if the database contains these three entries of class 'A':</p>
  
  <pre><code>A, named "one"
  A, named "two"
  A, named "three"
  </code></pre>
  
  <p>And two entries of class B:</p>
  
  <pre><code>B, points to "two"
  B, points to "three"
  </code></pre>
  
  <p>I want to select classes "two" and "three" of type A.</p>