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    <p>Here is what I do.</p> <p>I have a MigratingModel class, which all of my models inherit from. Here is migrating_model.py:</p> <pre><code>"""Models which know how to migrate themselves""" import logging from google.appengine.ext import db from google.appengine.api import memcache class MigrationError(Exception): """Error migrating""" class MigratingModel(db.Model): """A model which knows how to migrate itself. Subclasses must define a class-level migration_version integer attribute. """ current_migration_version = db.IntegerProperty(required=True, default=0) def __init__(self, *args, **kw): if not kw.get('_from_entity'): # Assume newly-created entities needn't migrate. try: kw.setdefault('current_migration_version', self.__class__.migration_version) except AttributeError: msg = ('migration_version required for %s' % self.__class__.__name__) logging.critical(msg) raise MigrationError, msg super(MigratingModel, self).__init__(*args, **kw) @classmethod def from_entity(cls, *args, **kw): # From_entity() calls __init__() with _from_entity=True obj = super(MigratingModel, cls).from_entity(*args, **kw) return obj.migrate() def migrate(self): target_version = self.__class__.migration_version if self.current_migration_version &lt; target_version: migrations = range(self.current_migration_version+1, target_version+1) for self.current_migration_version in migrations: method_name = 'migrate_%d' % self.current_migration_version logging.debug('%s migrating to %d: %s' % (self.__class__.__name__, self.current_migration_version, method_name)) getattr(self, method_name)() db.put(self) return self </code></pre> <p><code>MigratingModel</code> intercepts the conversion from the raw datastore entity to the full db.Model instance. If <code>current_migration_version</code> has fallen behind the class's latest <code>migration_version</code>, then it runs a series of <code>migrate_N()</code> methods which do the heavy lifting.</p> <p>For example:</p> <pre><code>"""Migrating model example""" # ...imports... class User(MigratingModel): migration_version = 3 name = db.StringProperty() # deprecated: use first_name and last_name first_name = db.StringProperty() last_name = db.StringProperty() age = db.IntegerProperty() invalid = db.BooleanProperty() # to search for bad users def migrate_1(self): """Convert the unified name to dedicated first/last properties.""" self.first_name, self.last_name = self.name.split() def migrate_2(self): """Ensure the users' names are capitalized.""" self.first_name = self.first_name.capitalize() self.last_name = self.last_name.capitalize() def migrate_3(self): """Detect invalid accounts""" if self.age &lt; 0 or self.age &gt; 85: self.invalid = True </code></pre> <p>On a busy site, the migrate() method should retry if <code>db.put()</code> fails, and possibly log a critical error if the migration didn't work.</p> <p>I haven't gotten there yet, but at some point I would probably mix-in my migrations from a separate file.</p> <h3>Final thoughts</h3> <p>It is hard to test on App Engine. It's hard to get access to your production data in a test environment, and at this time it is difficult-to-impossible to make a coherent snapshot backup. Therefore, for major changes, consider <strong>making a new version that uses a completely different model name</strong> which imports from the old model and migrates as it needs. (For example, <code>User2</code> instead of <code>User</code>). That way, if you need to fall back to the previous version, you have an effective backup of the data.</p>
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    1. CO+1 for Final Thoughts. It's Google, it's BigTable, use a new model name. Seems like the best solution.
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