I have a pretty standard use-case. I have a parent object and a list of child objects. I want to have a tabular form where I can edit all the children at once, as rows in th
Maybe you should just cheat. Put the new records in a different faux attribute that is a decorator for the actual one.
parent[children_attributes][0][fieldname]
parent[new_children_attributes][][fieldname]
It's not pretty, but it should work. It might take some extra effort to support round-trips to the form for validation errors.