'Determine new value for auto-updated property.
Some properies (e.g. DateTimeProperty, UserProperty) optionally update their
value on every put(). This call must return the new desired value for such
properties. For all other properties, this call must return
AUTO_UPDATE_UNCHANGED.
Args:
model_instance: Instance to get new value for.
Returns:
Datastore representation of the new model value in a form that is
appropriate for storing in the datastore, or AUTO_UPDATE_UNCHANGED.'

def get_updated_value_for_datastore(self, model_instance):

    return AUTO_UPDATE_UNCHANGED
