Node + Mongoose: Get last inserted ID?

I want to retrieve the last inserted _id, using mongoose as MongoDB wrapper for node.js. I've found the following tutorial, but I can't change any node modules because the app runs on a public server:

Getting "Last Inserted ID" (hint - you have to hack Mongoose)

Any other ideas? This what I want to do:

• Insert new user
• Get user's _id value
• Set a new session based on user's id
• Redirect to /

Thanks!