text: from google.appengine.ext import ndb
  
  class Teacher(ndb.Model):
      name = ndb.StringProperty(required=True)
      # email = ndb.StringProperty(required=True)
  
  class Course(ndb.Model):
      name = ndb.StringProperty(required=True)
      teachers = ndb.KeyProperty(Teacher,repeated=True)
  
      @classmethod
      def query_courses(cls, ancestor_key):
        return cls.query(ancestor=ancestor_key).order(cls.name)
  
  class User(ndb.Model):
      name = ndb.StringProperty(required=True)
      username = ndb.StringProperty(required=True)
      email = ndb.StringProperty(required=True)
  
  class Post(ndb.Model):
      author = ndb.KeyProperty(User, required=True)
      board = ndb.StringProperty(required=True)
      content = ndb.StringProperty(required=True)
      time = ndb.DateTimeProperty(auto_now_add=True)
  
  class Enrollment(ndb.Model):
      user = ndb.KeyProperty(User)
      course = ndb.KeyProperty(Course)
  