title: Issue with adding new properties to existing Google AppEngine data models / entities
body: <p>In GAE, I have a model called Foo, with existing entities, and attempt to add a new property called <em>memcached</em> to Foo that takes datetime values for the last time this value was set to memcache.  If I try to query and sort on this property, or even filter for entities that do not have a value for <em>memcached</em>, entities that haven't had a value set for this property yet are not returned.  Is there something I'm missing here, or as an alternative, is there a quick way to set a value for a <em>new</em> property on every entity of a given model?</p>
  
  <p>I have created a bunch of entities of the following model,</p>
  
  <pre><code>class Foo(db.Model):
      name = db.StringProperty(required=True)
  </code></pre>
  
  <p>and then add a property to this model,</p>
  
  <pre><code>class Foo(db.Model):
      name = db.StringProperty(required=True)
      memcached = db.DateTimeProperty(required=True, auto_now=True, auto_now_add=True, default=datetime.min)
  </code></pre>
  
  <p>the default value of the new property is not considered when I do a sort or filter on a query.</p>