A property to store serialized protorpc message models as Json data objectsin the appengine datastore, avoiding weird string key accessors from using aJsonProperty, and extra datastore hits using a StructuredProperty.

jsonproperty.py

'''A property to store serialized protorpc message models as Json data objects
in the appengine datastore, avoiding weird string key accessors from using a
JsonProperty, and extra datastore hits using a StructuredProperty.'''

from libs import ndb
from libs.protorpc.remote import protojson


class JsonMessageProperty(ndb.JsonProperty):

  _message_type = None

  # _attributes = ['_name', '_indexed', '_repeated', '_required', '_default',
  # '_choices', '_validator', '_verbose_name', '_message_type']

  def __init__(self, message_type=None, *args, **kwds):
    self._attributes.append('_message_type')
    self._message_type = message_type
    ndb.JsonProperty.__init__(self, *args, **kwds)

  def _default(self):
    return message_type()


  def _to_base_type(self, value):
    # decode message to json, store value in datastore
    return self.encode_message_to_json(value)

  def encode_message_to_json(self, value):
    return protojson.encode_message(value)


  def _from_base_type(self, value):
    # read from datastore, encode value to message object
    return self.encode_json_to_message(self._message_type, value)

  def encode_json_to_message(self, message_type, value):
    return protojson.decode_message(message_type, value)

usage.py

from google.appengine.ext import ndb
from jsonproperty import JsonMessageProperty
from protorpc import messages


class ModelMeta(messages.Message):
  created = messages.FloatField(1)
  edited = messages.FloatField(2)
  something_else = messages.StringField(3)


class SomeModel(ndb.Model):
  meta = JsonMessageProperty(message_type=ModelMeta)