Map-reduce task to clean up App Engine datastore entities whose models have changed

scrubber.py

from mapreduce import operation as op

def scrubber(entity):
    """Deletes any data from the underlying datastore that are no longer
    present in the entity's model definition.
    """
    # What properties are still defined on the model?
    valid_props = set(entity.properties().keys())
    # What properties are attached in the datastore?
    raw_props = set(entity._entity.keys())
    # We need to delete those that are in raw_props but not valid_props
    to_delete = raw_props - valid_props
    if to_delete:
        for field in to_delete:
            del entity._entity[field]
        yield op.db.Put(entity)