How can I serialize a MongoDB ObjectId with Marshmallow?

Question

I\'m building and API on top of Flask using marshmallow and mongoengine. When I make a call and an ID is supposed to be serialized I receive the following error:

Answer 1

You can extend the fields.Field class to create your own field. Here's how marshmallow-mongoengine (mentioned in another answer) implements this:

import bson
from marshmallow import ValidationError, fields, missing

class ObjectId(fields.Field):
    def _deserialize(self, value, attr, data):
        try:
            return bson.ObjectId(value)
        except Exception:
            raise ValidationError("invalid ObjectId `%s`" % value)

    def _serialize(self, value, attr, obj):
        if value is None:
            return missing
        return str(value)

and then:

class MySchema(Schema):
    id = ObjectId()

(I found this useful when not using MongoEngine, just using pymongo)