docs: def perform_index_validation(self, data):
          """
          Validate any unique indexes specified on the model.
          This should happen after all the normal fields have been validated.
          This can add error messages to multiple fields.
  
          :return: None
          """
          # Build a list of dict containing query values for each unique index.
          index_data = []
          for columns, unique in self.instance._meta.indexes:
              if not unique:
                  continue
              index_data.append({col: data.get(col, None) for col in columns})
  
          # Then query for each unique index to see if the value is unique.
          for index in index_data:
              query = self.instance.filter(**index)
              # If we have a primary key, need to exclude the current record from the check.
              if self.pk_field and self.pk_value:
                  query = query.where(~(self.pk_field == self.pk_value))
              if query.count():
                  err = ValidationError('index', fields=str.join(', ', index.keys()))
                  for col in index.keys():
                      self.add_error(col, err)