docs: def load_history(self):
          """Load upgrade history from database table.
  
          If upgrade table does not exists, the history is assumed to be empty.
          """
          if not self.history:
              query = Upgrade.query.order_by(desc(Upgrade.applied))
  
              for u in query.all():
                  self.history[u.upgrade] = u.applied
                  self.ordered_history.append(u.upgrade)