Hey, thanks for your work on this project. I was investigating integrating it with our EmberJS app but I ran into an issue. We have an Employee model that has a normal integer primary key, but also a username attribute. I'd like to only expose the username as the primary key, using DRF this is simple:

class EmployeeSerializer(serializers.ModelSerializer):
 id = serializers.ReadOnlyField(source="user.username")
 ...

However in build_json_resource_obj it just uses instance.pk:

def build_json_resource_obj(fields, resource, resource_instance, resource_name):
 resource_data = [
 ('type', resource_name),
 ('id', encoding.force_text(resource_instance.pk) if resource_instance else None),
 ...

Can you use the serializers ID field if available, and fallback to the instances PK?