title: How to add extra fields to Django Queryset
body: <p>I'd like to add some extra fields to my queryset.</p>
  
  <p>models.py</p>
  
  <pre><code>class Company(models.Model):
      ENTITY_CHOICES  = ( ('CO', 'Corporation'),('PR','Proprietor'),('PA','Partnership'),('NO','Nonprofit'))
      legal_name      = models.CharField(max_length=120, blank=True)
      entity          = models.CharField(max_length=1, null=True, choices=ENTITY_CHOICES, default='CO')
      client          = models.ForeignKey(Clients, models.SET_NULL, blank=True, null=True) 
  </code></pre>
  
  <p>views.py</p>
  
  <pre><code>qs = Company.objects.filter(entity=PA)
  for q in qs:
      q.due_date = '06/15'
  for q in qs:
      print('due_date',q.due_date)
  </code></pre>
  
  <p>It was successful in many times. But the result is not stable and I happen to see errors as follows,</p>
  
  <pre><code>Exception Type: AttributeError
  Exception Value:    
  'Companies' object has no attribute 'due_date'
  </code></pre>
  
  <p>Is there any better way to add extra attribute to querysets?</p>