Resaving Entities with Mapreduce

#mappers.py
from google.appengine.ext.mapreduce import operation as op
def resave(entity):
    ## computed fields will do the work.
    yield op.db.Put(entity)


####mapreduce.yaml
mapreduce:
- name: My Mapreduce
  mapper:
    input_reader: google.appengine.ext.mapreduce.input_readers.DatastoreInputReader
    handler: mappers.resave
    params:
    - name: entity_kind
      default: models.MyModel