docs: def _post_process_yaml_data(self,
                                  fixture_data: Dict[str, Dict[str, Any]],
                                  relationship_columns: Set[str],
                                  ) -> Tuple[Dict[str, Dict[str, Any]], List[str]]:
          """
          Convert and normalize identifier strings to Identifiers, as well as determine
          class relationships.
          """
          rv = {}
          relationships = set()
          if not fixture_data:
              return rv, relationships
  
          for identifier_id, data in fixture_data.items():
              new_data = {}
              for col_name, value in data.items():
                  if col_name not in relationship_columns:
                      new_data[col_name] = value
                      continue
  
                  identifiers = normalize_identifiers(value)
                  if identifiers:
                      relationships.add(identifiers[0].class_name)
  
                  if isinstance(value, str) and len(identifiers) <= 1:
                      new_data[col_name] = identifiers[0] if identifiers else None
                  else:
                      new_data[col_name] = identifiers
  
              rv[identifier_id] = new_data
          return rv, list(relationships)