Any time I personally ever want to generate JSON in Ruby, I want recursive tree traversal. It's just too painful to expect primitive types on everything. You'd have to pay that 50x doing it explicitly with a recursive function before you turn it into JSON anyway.

Ideally, you'd serialize directly from the database, bypassing the application entirely. Easily doable in ActiveRecord, but it's an explicit action, not the default. Not even sure if it's available in other databases besides PostgreSQL.