code: def dbreference_types(self):
          
          q = self.session.query(distinct(models.DbReference.type_))
          return [x[0] for x in q.all()]
docs: Distinct database reference types (``type_``) in :class:`.models.DbReference`
  
          :return: List of strings for all available database cross reference types used in model DbReference
          :rtype: list[str]