This seems to suggest you were looking for guidance or sane defaults for this. If that's not what you intended it to mean, I'm sorry for misreading it.
I am looking for guidance (or rather good discussion and ideas) about how to address this issue for larger Rails apps, just not from Rails or DHH. I don't think it's Rails' place to do this since I don't really think a one-size-fits-all approach is right and it sort of tramples over the core philosophy of ActiveRecord which I believe is a useful default. On the other hand I don't buy into DHH's philosophy that its all astronaut architecture either.