def _joined_ids(self):
        """
        Adds the subjects/visits used to generate the derivatives iff there are
        any joins over them in the pipeline
        """
        joined_prov = {}
        if self.joins_subjects:
            joined_prov['subject_ids'] = list(self.study.subject_ids)
        if self.joins_visits:
            joined_prov['visit_ids'] = list(self.study.visit_ids)
        return joined_prov