Index all values stored in the field, or only given ones if any.

def index(self, values=None, only_index=None):
        """
        Index all values stored in the field, or only given ones if any.
        """
        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