google app engine's ndb: get an entity's id

This looks simple, but I just didn't find how to get an entity's id from Google App Engine's ndb.

class Message(ndb.Model):
    name: ndb.StringProperty()
    ...

Create a message object:

message = Message(id=someId)
message.name = someName
message.put()

Later when I retrieve the entity:

message = Message.query(Message.name==someName).fetch(1)

Now how do I get the message's id? Thanks.