Question

I have a persistent class stored in a GAE datastore. I have removed one of the attributes from the class. The new records in this table show a value <none> for the removed attribute. But is there a way I can completely drop this column off the table?

Thanks.

Added the following ‘migration’ code according to moraes’ suggestion, but it isn’t achieving the desired result:

PersistenceManager pm = PMF.get().getPersistenceManager();
try {
    Query q = pm.newQuery(UserLogin.class);
    Collection<UserLogin> list = (Collection<UserLogin>) q.execute();

    Iterator<UserLogin> iter = list.iterator();
    while (iter.hasNext()) {
        UserLogin obj = (UserLogin) iter.next();
        obj.setLoginDate(obj.getLoginDate());
    }

    pm.makePersistentAll(list); 

} finally {
    pm.close();
}

Answer 1

I found the answer to this problem in this Article:
http://code.google.com/appengine/articles/update_schema.html

“Removing Deleted Properties from the Datastore

If you remove a property from your model, you will find that existing entities still have the property. It will still be shown in the admin console and will still be present in the datastore. To really clean out the old data, you need to cycle through your entities and remove the data from each one.

• Make sure you have removed the properties from the model definition.

• If your model class inherits from db.Model, temporarily switch it to inherit from db.Expando. (db.Model instances can’t be modified dynamically, which is what we need to do in the next step.)

• Cycle through existing entities (like described above). For each entity, use delattr to delete the obsolete property and then save the entity.

• If your model originally inherited from db.Model, don’t forget to change it back after updating all the data.”

And here is an example with code:
http://sandrylogan.wordpress.com/2010/12/08/delattr/