title: Django: One-to-many relation for [0, 1] cardinality
body: <p>Imagine having two models:</p>
  
  <pre><code>class Service(models.Model):
    key_service_name = models.ForeignKey(Key_service_type, related_name='Service_key_service_names', null=False)
    service_hourly_wage_rate = models.DecimalField(max_digits=5, decimal_places=2, null=True)
    service_amount = models.DecimalField(max_digits=7, decimal_places=2, null=True)
  
  class ServiceAdditionalInfo(models.Model):
    service = models.ForeignKey(Service, related_name='Service_additional_info_services', null=False)
    fixed_price = models.DecimalField(max_digits=8, decimal_places=2, null=True)
  </code></pre>
  
  <p>Amongst other info, the Service class serializer features this field description:</p>
  
  <pre><code>service_additional_info = ServiceAdditionalInfoSerializer(read_only = True, many=False, source = 'Service_additional_info_services')
  </code></pre>
  
  <p>In practice, one Service instance may be referenced by 0 or 1 ServiceAdditionalInfo instance.
  The serializer understandably returns a list while I would definitely prefer a dictionary.</p>
  
  <p>My question: Is this the recommended way of modelling the relation?
  If so, is there a django-built-in mechanism to return a dict for such cases?</p>
  
  <p>For the latter case I know how to work around the issue, but since I would like to use the framework in the intended way, I'm very curious if there's something I missed regarding modelling and serializers.</p>