How to serialize db.Model objects to json?

Question

When using

from django.utils import simplejson

on objects of types that derive from db.Model it throws exceptions. How to circumvent this?

Answer 1

Ok - my python not great so any help would be appreciated - You dont need to write a parser - this is the solution:

add this utlity class http://code.google.com/p/google-app-engine-samples/source/browse/trunk/geochat/json.py?r=55

 import datetime  
 import time 

 from google.appengine.api import users 
 from google.appengine.ext import db 

#this is a mod on the orinal file for some reason it includes its own simplejson files i have ref django!
 from django.utils import simplejson  

 class GqlEncoder(simplejson.JSONEncoder): 

   """Extends JSONEncoder to add support for GQL results and properties. 

   Adds support to simplejson JSONEncoders for GQL results and properties by 
   overriding JSONEncoder's default method. 
   """ 

   # TODO Improve coverage for all of App Engine's Property types. 

   def default(self, obj): 

     """Tests the input object, obj, to encode as JSON.""" 

     if hasattr(obj, '__json__'): 
       return getattr(obj, '__json__')() 

     if isinstance(obj, db.GqlQuery): 
       return list(obj) 

     elif isinstance(obj, db.Model): 
       properties = obj.properties().items() 
       output = {} 
       for field, value in properties: 
         output[field] = getattr(obj, field) 
       return output 

     elif isinstance(obj, datetime.datetime): 
       output = {} 
       fields = ['day', 'hour', 'microsecond', 'minute', 'month', 'second', 
           'year'] 
       methods = ['ctime', 'isocalendar', 'isoformat', 'isoweekday', 
           'timetuple'] 
       for field in fields: 
         output[field] = getattr(obj, field) 
       for method in methods: 
         output[method] = getattr(obj, method)() 
       output['epoch'] = time.mktime(obj.timetuple()) 
       return output 

     elif isinstance(obj, time.struct_time): 
       return list(obj) 

     elif isinstance(obj, users.User): 
       output = {} 
       methods = ['nickname', 'email', 'auth_domain'] 
       for method in methods: 
         output[method] = getattr(obj, method)() 
       return output 

     return simplejson.JSONEncoder.default(self, obj) 


 def encode(input): 
   """Encode an input GQL object as JSON 

     Args: 
       input: A GQL object or DB property. 

     Returns: 
       A JSON string based on the input object.  

     Raises: 
       TypeError: Typically occurs when an input object contains an unsupported 
         type. 
     """ 
   return GqlEncoder().encode(input)   

save as json.py

TO USE

import cgi
import os
import json 

from google.appengine.ext.webapp import template
from google.appengine.api import users
from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app
from google.appengine.ext import db


class Greeting(db.Model):
    author = db.UserProperty()
    content = db.StringProperty(multiline=True)
    date = db.DateTimeProperty(auto_now_add=True)

class MainPage(webapp.RequestHandler):
    def get(self):
        greetings_query = Greeting.all().order('-date')
        greetings = greetings_query.fetch(5)

        if users.get_current_user():
            url = users.create_logout_url(self.request.uri)
            url_linktext = 'Logout'
        else:
            url = users.create_login_url(self.request.uri)
            url_linktext = 'Login'

        template_values = {
            'greetings': greetings,
            'url': url,
            'url_linktext': url_linktext,
            }

        path = os.path.join(os.path.dirname(__file__), 'index.html')
        self.response.out.write(template.render(path, template_values))


class Guestbook(webapp.RequestHandler):
    def post(self):
        greeting = Greeting()

        if users.get_current_user():
            greeting.author = users.get_current_user()

        greeting.content = self.request.get('content')
        greeting.put()
        self.redirect('/')



#here i return my json feed - simple implementaion for example
class FeedHandler(webapp.RequestHandler):

  def get(self):
    """Retrieve a feed"""
    user = None

    greetings_query = Greeting.all().order('-date')
    rs= greetings_query.fetch(5)
#this is the part that calls the encoder - dosnt cause an exception
    data = json.encode(rs)



#roll out to browser -might need to check my headers etc
    self.response.headers['Content-Type'] = 'application/json; charset=utf-8'  
    self.response.out.write(data)




application = webapp.WSGIApplication(
                                       [
                                       ('/', MainPage),
                                       ('/sign',Guestbook),
                                       ('/feed',FeedHandler),
                                       ], debug=True
                                    )

def main():
    run_wsgi_app(application)

if __name__ == "__main__":
    main()

This is the browser response:

[{"content": "", "date": {"ctime": "Sat Jan 23 02:40:22 2010", "hour": 2, "isoweekday": 6, "month": 1, "second": 22, "microsecond": 434000, "isocalendar": [2010, 3, 6], "timetuple": [2010, 1, 23, 2, 40, 22, 5, 23, -1], "year": 2010, "epoch": 1264214422.0, "isoformat": "2010-01-23T02:40:22.434000", "day": 23, "minute": 40}, "author": {"nickname": "[email protected]", "email": "[email protected]", "auth_domain": "gmail.com"}}, {"content": "", "date": {"ctime": "Sat Jan 23 01:12:43 2010", "hour": 1, "isoweekday": 6, "month": 1, "second": 43, "microsecond": 972000, "isocalendar": [2010, 3, 6], "timetuple": [2010, 1, 23, 1, 12, 43, 5, 23, -1], "year": 2010, "epoch": 1264209163.0, "isoformat": "2010-01-23T01:12:43.972000", "day": 23, "minute": 12}, "author": {"nickname": "[email protected]", "email": "[email protected]", "auth_domain": "gmail.com"}}, {"content": "test", "date": {"ctime": "Fri Jan 22 22:32:13 2010", "hour": 22, "isoweekday": 5, "month": 1, "second": 13, "microsecond": 659000, "isocalendar": [2010, 3, 5], "timetuple": [2010, 1, 22, 22, 32, 13, 4, 22, -1], "year": 2010, "epoch": 1264199533.0, "isoformat": "2010-01-22T22:32:13.659000", "day": 22, "minute": 32}, "author": {"nickname": "[email protected]", "email": "[email protected]", "auth_domain": "gmail.com"}}, {"content": "", "date": {"ctime": "Fri Jan 22 22:29:49 2010", "hour": 22, "isoweekday": 5, "month": 1, "second": 49, "microsecond": 358000, "isocalendar": [2010, 3, 5], "timetuple": [2010, 1, 22, 22, 29, 49, 4, 22, -1], "year": 2010, "epoch": 1264199389.0, "isoformat": "2010-01-22T22:29:49.358000", "day": 22, "minute": 29}, "author": {"nickname": "[email protected]", "email": "[email protected]", "auth_domain": "gmail.com"}}, {"content": "ah it works!\r\n", "date": {"ctime": "Fri Jan 22 22:29:22 2010", "hour": 22, "isoweekday": 5, "month": 1, "second": 22, "microsecond": 995000, "isocalendar": [2010, 3, 5], "timetuple": [2010, 1, 22, 22, 29, 22, 4, 22, -1], "year": 2010, "epoch": 1264199362.0, "isoformat": "2010-01-22T22:29:22.995000", "day": 22, "minute": 29}, "author": {"nickname": "[email protected]", "email": "[email protected]", "auth_domain": "gmail.com"}}]