Quering a document using an instance of ObjectId does not return a result even if the document exists.

Try this in python console:

import pymongo
from bson import ObjectId
client = pymongo.MongoClient('localhost', 27017)
db = client.local

db.test.insert(

{"_id": "51c9d99649b5d534db8d483a", "var": "foo"}

)

try:
doc = db.test.find_one(

{'_id': ObjectId('51c9d99649b5d534db8d483a')}

)
assert (doc != None)
except:
print 'Document not found!'

try:
doc = db.test.find_one(

{'_id': str(ObjectId('51c9d99649b5d534db8d483a'))}

)
assert (doc != None)
except:
print 'Document not found!'

Using the string representation of ObjectId returns the spected result.