Insert python object in mongodb

from pymongo import Connection
import json

conn = Connection()
db = conn.cl_database
postings = db.postings_collection

class Posting(object):
def __init__(self, link, found=None, expired=None):
self.link = link
self.found = found
self.expired = expired

posting = Posting('objectlink1')
value = json.dumps(posting, default=lambda x:x.__dict__)
postings.insert(value)

Traceback (most recent call last):
File "./mongotry.py", line 21, in <module>
postings.insert(value)
File "build/bdist.macosx-10.7-intel/egg/pymongo/collection.py", line 302, in insert
File "build/bdist.macosx-10.7-intel/egg/pymongo/database.py", line 252, in _fix_incoming
File "build/bdist.macosx-10.7-intel/egg/pymongo/son_manipulator.py", line 73, in transform_incoming
TypeError: 'str' object does not support item assignment

posting = Posting('objectlink1')
value = json.dumps(posting, default=lambda x:x.__dict__)
value = json.loads(value)
postings.insert(value)