Getting 'TypeError: ObjectId('') is not JSON serializable' when using Flask 0.10.1

Question

I forked the Flask example, Minitwit, to work with MongoDB and it was working fine on Flask 0.9, but after upgrading to 0.10.1 I get the error in title when I login when I try t

Answer 1

JSON only supports serializing (encoding/decoding) a limited set of objects types by default. You could extend python JSON's decoder/encoder's to handle this situation though.

In terms of encoding an object which contains on ObjectID, for example, when ObjectIds are created client side, which will be passed along to some awaiting server, try:

import json
from bson.objectid import ObjectId

class Encoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, ObjectId):
            return str(obj)
        else:
            return obj

Then, in your code, before pushing the data client -> server, run:

json.dumps(obj, cls=Encoder)

Server side, if we know we're dealing with mongo docs, (dictionary object with an '_id' key), we can define a json decoder hook like the following:

def decoder(dct):
    for k, v in dct.items():
        if '_id' in dct:
            try:
                dct['_id'] = ObjectId(dct['_id'])
            except:
                pass
        return dct

And call it using a call like the following:

doc = json.loads(in_doc, object_hook=decoder)

You'll probably need to adapt this code a bit, but for the simple case of passing