text: from google.appengine.ext import ndb
  
  class ChatMessage(ndb.Model):
      """Single Message send by user to chat room"""
      user= ndb.StringProperty(required=True)
      timestamp=ndb.DateTimeProperty(auto_now_add=True)
      message=ndb.TextProperty(required=True)
      def __str__(self):
          return "From %s at %s : %s " %(self.user,self.timestamp,self.message)
      
  class ChatTopic(ndb.Model):
      createdBy=ndb.StringProperty(required=True)
      topicName=ndb.StringProperty(required=True)
  #     messages = ndb.ListProperty(ChatMessage)