http://code.google.com/appengine/docs/datastore/transactions.html#Uses_For_Transactions
>> Another common use for transactions is to update an entity with a named
>> key, or create it if it doesn't yet exist:
>>
>> class SalesAccount(db.Model):
>> address = db.PostalAddressProperty()
>> phone_number = db.PhoneNumberProperty()
>>
>> def create_or_update(parent_obj, account_id, address, phone_number):
>> obj = db.get(Key.from_path("SalesAccount", account_id, parent=parent_obj))
>> if not obj:
>> obj = SalesAccount(parent=parent_obj,
>> address=address,
>> phone_number=phone_number)
>> else:
>> obj.address = address
>> obj.phone_number = phone_number
>>
>> obj.put()
>>
>> As before, a transaction is necessary to handle the case where another user
>> is attempting to create or update an entity with the same account_id.
>> Without a transaction, if the entity does not exist and two users attempt
>> to create it, the second will fail. With a transaction, the second attempt
According to Paths and Key Uniqueness(http://code.google.com/appengine/
docs/datastore/keysandentitygroups.html#Paths_and_Key_Uniqueness),
just same kind and same parent do no mean same key, since the ids or
key names differ. Without the same key, obj2 should be put without
clash, even with same properties of obj1. Thus, the second should not
fail even without a transaction.
>> will retry, notice that the entity now exists, and update the entity
>> instead.
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