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 circ

Answer 1

From what i can understand - and i am new to python - with google app engine the work around is to serialize the model object to a dictioanry python object and then use simple json to dump it as a json string - this makes no sense to me - maybe someone has the know to serialise to a ditionary (pickel?) Any help on this would be HELP! not to impressed that google app engine has no inbuilt solution for this.

Answer 2

The example provided by Jader Dias works fine for my concern after some twaeking. Remove the encode method as it contains a circular reference. The adjusted class should look like:

import datetime  
import time 

from google.appengine.api import users 
from google.appengine.ext import db 
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, datetime.date): 
            output = {} 
            fields = ['year', 'month', 'day'] 
            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) 

As I've saved this class in a file called utils.py and when appropriate I import it using

import utils

Then I just call utils.GqlEncoder().encode(results), for example:

query = User.all()
results = query.fetch(10)

self.response.headers['Content-Type'] = "text/plain" # Alt. application/json
self.response.out.write( utils.GqlEncoder().encode(results) )

The result should look something like this (I've added some line feeds in order to make it a bit easier to read):

[
{"date": {"ctime": "Tue Feb 23 10:41:21 2010", "hour": 10, "isoweekday": 2, "month": 2, 
        "second": 21, "microsecond": 495535, "isocalendar": [2010, 8, 2], "timetuple": [2010, 2, 23, 10, 41, 21, 1, 54, -1], 
        "year": 2010, "epoch": 1266921681.0, "isoformat": "2010-02-23T10:41:21.495535", "day": 23, "minute": 41}, 
"claimed_id": "https:\/\/www.google.com\/accounts\/o8\/id?id=abcdefghijklmnopqrstuvxyz", 
"display_name": "Alfred E Neumann", 
"email": null, 
"full_name": "Alfred E Neumann"
}, 
{"date": {"ctime": "Tue Feb 23 11:00:54 2010", "hour": 11, "isoweekday": 2, "month": 2, 
        "second": 54, "microsecond": 805261, "isocalendar": [2010, 8, 2], "timetuple": [2010, 2, 23, 11, 0, 54, 1, 54, -1], 
        "year": 2010, "epoch": 1266922854.0, "isoformat": "2010-02-23T11:00:54.805261", "day": 23, "minute": 0}, 
"claimed_id": "http:\/\/openid.domain.net\/john", 
"display_name": "", 
"email": "jp@domain.net", 
"full_name": "John Parnefjord"
}
]

Answer 3

Since I could not find an appropriate solution I wrote my own, which is not exactly a JSON serializer, but a Javascript serializer

from google.appengine.ext import db
from google.appengine.api.datastore_types import *

def dumpStr(obj):
    return "'" + obj + "'"

def dumps(obj):
    if isinstance(obj, str):
        return dumpStr(obj)
    elif obj == None:
        return None
    elif isinstance(obj, list):
        items = [];
        for item in obj:
            items.append(dumps(item))
        return '[' + ','.join(items) + ']'
    elif isinstance(obj, datetime.datetime):
        return "new Date('%s')" % obj.ctime()
    properties = [];
    for property in dir(obj):
        if property[0] != '_':
            value = obj.__getattribute__(property)
            valueClass = str(value.__class__)
            if not(('function' in valueClass) or ('built' in valueClass) or ('method' in valueClass)):
                value = dumps(value)
                if value != None:
                    properties.append("'" + property + "':" + value)
    if len(properties) == 0:
        return str(obj)
    else:
        return '{' + ','.join(properties) + '}'

Answer 4

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": "test@example.com", "email": "test@example.com", "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": "test@example.com", "email": "test@example.com", "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": "test@example.com", "email": "test@example.com", "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": "test@example.com", "email": "test@example.com", "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": "test@example.com", "email": "test@example.com", "auth_domain": "gmail.com"}}]

Answer 5

json cannot be used to serialize anything more than basic types such as dicts, lists, ints/longs, and strings (this is not comprehensive). For example, even these simple commands do not work:

import json
json.dumps(object())

If you want to serialize django objects, you should refer to the django documentation on serialization, which will use their own libraries, but they do do support json.