title: django serialize exclude primary key for some reason
body: <p>My model is as following:</p>
  
  <pre><code>class PolicyRule(models.Model):
      uid = models.IntegerField(default=0, primary_key=True, unique=True)
      maxAmount = models.FloatField()
      destinations = models.TextField()
  </code></pre>
  
  <p>and for some reason, the following code</p>
  
  <pre><code>rules_list = PolicyRule.objects.all()
  paginator = Paginator(rules_list, 5)
  rules = paginator.page(page)
  rules_json = serialize('json', list(rules))
  </code></pre>
  
  <p>produces this output
  <code>
  "[{\"model\": \"webapp.policyrule\", \"pk\": 1576966788, \"fields\": {\"maxAmount\": 50.0, \"destinations\": \"ronen\"}}]"
  </code></p>
  
  <p>which is without the UID field! why is this happening?</p>