Question

I’ve changed my object to have a new required property in v2. When I attempt to fetch a v1 object from the datastore, I get BadValueError because v1 doesn’t have the required property. What’s the best way to introduce new required properties on existing data

Answer 1

I would resolve this problem using the mapreduce library.

First, register the mapper in mapreduce.yaml:

mapreduce:
- name: fixing required property
  mapper:
    input_reader: mapreduce.input_readers.DatastoreInputReader
    handler: your handler
    params:
    - name: entity_kind
      default: main.ModelV2

then define a process function to modify the entities:

from mapreduce import operation as op
def process(entity):
 if not entity.newproperty :
    entity.newproperty = None
 yield op.db.Put(entity)

If you are dealing with a relative small number of entities, you could avoid mapreduce modifying directly your entities with something like this:

entities = ModelV2.all()
for entity in entities :
  if not entity.newproperty :
    entity.newproperty = None
    entity.put()