Now on the case with http://apidock.com/rails/v4.2.1/ActionView/Helpers/FormHelper/fields_for (yes, I know, Rails 4, probably isn’t supported in 5) and mentions more possibilities that probably aren’t relevant, but don’t work anyway.
I mention it here because if (I’m not confident enough to say when) I get this working I will be needing to auto-populate some fields (am I mad!) and there might be value in the comments.
It’s another one of those times when it’s clear that I’m doing something wrong (other than actually attempting to do this) but I have no idea what and very little indication of where to start looking. For all the times that development can accelerate with Rails, there are these occasions when it grinds to a halt and you’re never actually sure how it got fixed.