Just the basics of MongoDB with Ruby:
The MongoDB Ruby driver is not only simple to use, but it will get you familiar with how queries look and how they operate. Armed with this knowledge, moving into an ORM becomes much easier. You’ll not only be able to understand what is abstracted away, but you’ll be able to spot bad and inefficient generated queries, making performance troubleshooting a snap.
Original title and link: Being Awesome with the MongoDB Ruby Driver (NoSQL databases © myNoSQL)