JSON serialization of Google App Engine models
In the latest (1.5.2) release of the App Engine SDK, a to_dict()
function that converts model instances to dictionaries was introduced in db.py
. See the release notes.
There is no reference to this function in the documentation as of yet, but I have tried it myself and it works as expected.
A simple recursive function can be used to convert an entity (and any referents) to a nested dictionary that can be passed to simplejson
:
import datetime
import time
SIMPLE_TYPES = (int, long, float, bool, dict, basestring, list)
def to_dict(model):
output = {}
for key, prop in model.properties().iteritems():
value = getattr(model, key)
if value is None or isinstance(value, SIMPLE_TYPES):
output[key] = value
elif isinstance(value, datetime.date):
# Convert date/datetime to MILLISECONDS-since-epoch (JS "new Date()").
ms = time.mktime(value.utctimetuple()) * 1000
ms += getattr(value, 'microseconds', 0) / 1000
output[key] = int(ms)
elif isinstance(value, db.GeoPt):
output[key] = {'lat': value.lat, 'lon': value.lon}
elif isinstance(value, db.Model):
output[key] = to_dict(value)
else:
raise ValueError('cannot encode ' + repr(prop))
return output
This is the simplest solution I found. It requires only 3 lines of codes.
Simply add a method to your model to return a dictionary:
class DictModel(db.Model):
def to_dict(self):
return dict([(p, unicode(getattr(self, p))) for p in self.properties()])
SimpleJSON now works properly:
class Photo(DictModel):
filename = db.StringProperty()
title = db.StringProperty()
description = db.StringProperty(multiline=True)
date_taken = db.DateTimeProperty()
date_uploaded = db.DateTimeProperty(auto_now_add=True)
album = db.ReferenceProperty(Album, collection_name='photo')
from django.utils import simplejson
from google.appengine.ext import webapp
class PhotoHandler(webapp.RequestHandler):
def get(self):
photos = Photo.all()
self.response.out.write(simplejson.dumps([p.to_dict() for p in photos]))