title: Django ORM: pre-filtering related fields inside a queryset
body: <p>Imagine I have two models, <code>A</code> and <code>B</code>, which are defined (loosely) as</p>
  
  <pre><code>class A(models.Model):
      a_flag = models.BooleanField()
  
  class B(models.Model):
      b_flag = models.BooleanField()
      a = models.ForeignKey('A', ...)
  </code></pre>
  
  <p>I want to have a queryset of <code>A</code>'s which have <code>a_flag=True</code>, and for these, I want further queries to <code>a.b.filter(...)</code> be kind of 'pre-filtered' with <code>b_flag=True</code>. Is there a way of doing this without redefining the RelatedManager of <code>B</code> (still seems like a hacky solution)?</p>
  
  <p>UPD: foreign key was the other way around</p>