[huggyface]

Storing an entire notebook in a single document would be the most obvious.

The cost of course being that a change to any note in a notebook yields a save of the entire document. Not a problem in simple cases, but that sort of mass-write-amplification can kill you (talk to Digg about that).

Also - I have found a hybrid between nosql (mongodb) and sql (postgres) is ideal - who says you need to use a single database?

Simplicity. Coherency. Maintainability. And on. Sure, it might make sense, but if you already have you toes in the "SQL" world, it is usually worthwhile to dunk your whole foot in. Many SQL products also offer the document functionality of MongoDB, for instance. SQL Server, as an example, lets you store XML documents to your hearts content, which you can index and intelligently query upon, etc. Your scheme is boundless, and on and on.