title: is it possible to make a query lookup for given model inheritance?
body: <p>Below are my models and I am using <code>InheritanceManager</code> of <code>django-model-utils</code>. i decided the field <code>user</code> to be in the subclasses because i need to access the <code>Child</code> subclass via the relation like <code>user.child1_set.all()</code>.</p>
  
  <pre><code>class Mother(models.Model):
      objects = InheritanceManager()
  
  class Child1(Mother):
      user = models.ForiegnKey(User)
  
  class Child2(Mother):
      user = models.ForiegnKey(User)
  </code></pre>
  
  <p>the problem is, when i want to query starting from <code>Mother</code> class, there seems to be no query that makes it.</p>
  
  <p>i've tried these codes.</p>
  
  <pre><code>Mother.objects.filter(user=SOMEUSER)
  Mother.objects.filter(child1__user=SOMEUSER)
  Mother.objects.select_subclasses().filder(user=SOMEUSER)
  Mother.objects.select_subclasses().filder(child1__user=SOMEUSER)
  </code></pre>
  
  <p>any advice would be appreciated.</p>