alter PolyModel inheritance tree

migration.py

from logging import debug as log_debug
from google.appengine.ext import deferred
from google.appengine.ext import ndb

from model2 import Child #  this works for query purposes!
# deserialize bigtable data into object will yield
# just GrandParent fields prior to migration running

BATCH_SIZE = 20

PARENT_CLASS_NAME = u'Parent'

# based on code found here:
# https://cloud.google.com/appengine/articles/update_schema


def add_parent_class(cursor=None, num_updated=0):
    # call this method to convert from model1 to model2
    query = Child.all() #  get all existing entities to migrate
    if cursor:
        query.with_cursor(cursor)

    to_put = []
    for entity in query.fetch(limit=BATCH_SIZE):
        # class_ field mutation allowed, assignment is not
        entity.class_.insert(1, PARENT_CLASS_NAME)
        to_put.append(entity)

    if to_put:
        ndb.put_multi(to_put) #  fewer RPCs this way
        num_updated += len(to_put)
        log_debug('updated %d entities. for a total of %d',
            len(to_put), num_updated)
        deferred.defer(add_parent_class,
            cursor=query.cursor(), num_updated=num_updated)
    else:
        log_debug('add_parent_class complete with %d updates',
            num_updated)


def remove_parent_class(cursor=None, num_updated=0):
    # call this method to convert from model2 to model1
    query = Child.all() #  get all existing entities to migrate
    if cursor:
        query.with_cursor(cursor)

    to_put = []
    for entity in query.fetch(limit=BATCH_SIZE):
        # class_ field mutation allowed, assignment is not
        entity.class_.remove(PARENT_CLASS_NAME)
        to_put.append(entity)

    if to_put:
        ndb.put_multi(to_put) #  fewer RPCs this way
        num_updated += len(to_put)
        log_debug('updated %d entities. for a total of %d',
            len(to_put), num_updated)
        deferred.defer(add_parent_class,
            cursor=query.cursor(), num_updated=num_updated)
    else:
        log_debug('remove_parent_class complete with %d updates',
            num_updated)

model1.py

from google.appengine.ext import ndb
from google.appengine.ext.ndb import polymodel


class GrandParent(polymodel.PolyModel):
    field1 = ndb.StringProperty()


class Child(GrandParent):
    field2 = ndb.StringProperty()

model2.py

from google.appengine.ext import ndb
from google.appengine.ext.ndb import polymodel


class GrandParent(polymodel.PolyModel):
    field1 = ndb.StringProperty()


class Parent(GrandParent):
    field3 = ndb.StringProperty()


class Child(Parent):
    field2 = ndb.StringProperty()