An upsert into the "local.system.users" collection creates documents with no _id. Since there is an index on _id, this will create an index key of

for the first upserted user document. So upserting another user document will fail with the following error.

E11000 duplicate key error index: local.system.users.$id dup key: { : null }

This happens only if local system.users is empty or there are no documents with _id set. If there is at least one document with _id set then it will work fine.

Since pymongo adds users using upserts, this will make pymongo fail when adding more than one local user

To replicate:
1- start a fresh 2.2.0 mongod server(nuke dbpath)

1. mongod --dbpath /data/db --port 27017

2- mongo into server and use local db

1. mongo

3- upsert a user document into system.users

1. db.system.users.update( {"user":"abdul"}

   ,

   {"user":"abdul", "pwd":"xxxxxxxxx"}

   , true);

4- Note that inserted document does not have an _id

1. db.system.users.find()

5- Insert another document

RESULT:

E11000 duplicate key error index: local.system.users.$id dup key: { : null }