@janpet @kaspth I've checked your questions, and the one I'm sure about is that:

Yes, you always have to initialize a Report (parent class) and a CoverSheet (child class). You can't just initilalize a child class on its own. I've tried it, doesn't work (well, you can make it work, but at what cost 😅).

I remember hitting a `ActiveRecord::HasManyThroughNestedAssociationsAreReadonly` but I can't remeber for the life of me how i solved this one.

@janpet I wrote about some of the pitfalls I encountered when working with DT in the past: https://remimercier.com/delegated-types/ It might help! Yell if it doesn't!

Also @kaspth just wrote something of his own here: https://kaspth.com/posts/making-rails-delegated_types-clearer More about making the API better, but it gives some good knowledge on the design.