code: def index(self, values=None, only_index=None):
          
          assert self.indexable, "Field not indexable"
          assert not only_index or self.has_index(only_index), "Invalid index"
          if only_index:
              only_index = only_index if isclass(only_index) else only_index.__class__
  
          if values is None:
              values = self.proxy_get()
  
          for value in values:
              if value is not None:
                  needs_to_check_uniqueness = bool(self.unique)
  
                  for index in self._indexes:
                      if only_index and not isinstance(index, only_index):
                          continue
  
                      index.add(value, check_uniqueness=needs_to_check_uniqueness and index.handle_uniqueness)
  
                      if needs_to_check_uniqueness and index.handle_uniqueness:
                          # uniqueness check is done for this value
                          needs_to_check_uniqueness = False
docs: Index all values stored in the field, or only given ones if any.