new_contents: from google.appengine.ext import ndb
  import json
  
  class Timetable(ndb.Model):
      for_class = ndb.StringProperty(indexed=True)
      plan = ndb.StringProperty(indexed=False)
  
      @classmethod
      def get_for_class(cls, for_class):
          return cls.query(cls.for_class == for_class)
  
      @classmethod
      def get_all(cls):
          return cls.query()
  
      def to_dict(self):
          selfdict = dict(
              id=self.key.urlsafe(),
              for_class=self.for_class
          )
          try:
              selfdict['plan'] = json.loads(self.plan)
          except ndb.UnprojectedPropertyError:
              pass
          return selfdict
  
subject: add Timetable model