title: Adding extra fields to a QuerySet that spans a relation
body: <p>I'm trying to do something like this:</p>
  
  <pre><code>class A(models.Model):
      members = models.ManyToManyField(B)
  
  class B(models.Model):
      pass
  
  # not sure what the right query is here
  results = A.objects.all().[members.extra(select={"extra_field": ...})?]
  
  # I want to be able to write this using the result of my query:
  for r in result:
      for m in r.members:
          print m.extra_field
  </code></pre>
  
  <p>Is it possible to populate this <code>extra_field</code> without creating a new query for every model B in members?</p>