docs: def clean_for_storage(self, data):
          """Clean data in preparation for storage.
  
          Deletes items with key having a '.' or is '_id'. Also deletes those items
          whose value is a dictionary or a list.
  
          Args:
              data: Sample data dictionary to be cleaned.
  
          Returns:
              Cleaned data dictionary.
          """
          data = self.data_to_unicode(data)
          if isinstance(data, dict):
              for k in dict(data).keys():
                  if k == '_id':
                      del data[k]
                      continue
                  if '.' in k:
                      new_k = k.replace('.', '_')
                      data[new_k] = data[k]
                      del data[k]
                      k = new_k
                  if isinstance(data[k], dict):
                      data[k] = self.clean_for_storage(data[k])
                  elif isinstance(data[k], list):
                      data[k] = [self.clean_for_storage(item) for item in data[k]]
          return data