I noticed that fields_for suffixed the nested attributes key with _attributes. This makes it incompatible with ORMs that accept nested attributes (ex: DataMapper). Ideally, no suffix should be added.
This might have stayed there because of ActiveRecord, since that's the way it handles nested attributes. I agree with you that it should be changed, particularly if you're dealing with other ORMs that support direct nesting of relations. I'm working on a patch for this and will push it for you to review it as soon as I can.
Looks like I have to use dm-accepts_nested_attributes, which adds _attributes= methods.
That's definitely an easier solution. I've added the explanation to the docs here. Perhaps we should think of adding it for the rest of the ORMs at some point...