If I define a nested model or a relation in a mongoose schema, does it have a mechanism to autoload everything when re-hydrating instances, or do I simply have to ensure I've always loaded the appropriate models?
Nested, embedded model docs are auto-loaded, but if it's an ObjectId based relation, then you need to use Mongoose's populate support to explicitly load the related docs.
Mongoose performs all these actions based on the schemas and models that your code defines, typically during your app's startup.