code: def get_natural_key_fields(cls):
          
          natural_key = []
          for name, rel_to in cls.get_natural_key_info():
              if not rel_to:
                  natural_key.append(name)
              else:
                  nested_key = rel_to.get_natural_key_fields()
                  natural_key.extend([
                      name + '__' + nname
                      for nname in nested_key
                  ])
          return natural_key
docs: Determine actual natural key field list, incorporating the natural keys
          of related objects as needed.