ActiveSupport also monkey-patches to_json with a recursive Ruby function, completely nerfing JSON performance by 50x in a lot of cases. Unfortunately, it's what makes 'render json: @model' just... work.
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.
Have you ever used Ruby's JSON.dump? The second it runs into anything that it can't convert, you get "#<Object:0x00007fab05133078>" everywhere in your output.
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.