models.py

class MyModel(PolymorphicModel):

    @property
    def driver(self):
        if self.is_departure: return self.delivery_driver
        else: return self.collection_driver
    @driver.setter
    def driver(self, value):
        if self.is_departure: self.delivery_driver = value
        else self.collection_driver = value
        

serializers.py

class MyModelSerializer(serializers.ModelSerializer):
  
  driver = UserSerializer()