Question

On Google App Engine, I would like to create a unique key for a data table. As suggested by this, I am trying to do the following:

Suppose I have a model called Message, and the field values are obtained from a web page as follows:

message = db.Message()
message.name = self.request.get('name')
message.email = self.request.get('email')
...
message.put()
message.messageId = message.key().id()
message.put()

I have two questions:

1. Is this the correct way of creating a unique id (messageId)? I mean: put the entity first, get it’s id and assign to messageId, and finally put again? It looks strange.

2. How to prevent the concurrency problem? I know that I can use ‘transaction’ as follows:

   @db.transactional
   def storeEntity():
       message.name = self.request.get('name')
       message.email = self.request.get('email')
       ...
       message.put()
       message.messageId = message.key().id()
       message.put()
   

But what if this transaction fails? How do I do it until it is successful? Thanks in advance.

Answer 1

When you do not provide a key, the datastore will return a key for you, so you do not have to put the entity twice. And put will return the key.

So you can :

message_key = Message(name = self.request.get('name'), email : self.request.get('email')).put()