Based on what Jonathan and I were able to reconstruct from memory about the IRC discussion around this issue, the actual use case here is a form that has multiple inputs, which map to a single Item attribute, a reference to another Item which has the values of those inputs set on it. Unfortunately the original bug description doesn't cover this at all; I've often found it very useful to be more specific about the concrete use case that motivates a more general change, even if the benefits are "obvious".
Anyhow, I think this is the wrong way of going about things. A single item attribute should be associated with a single model parameter, which in turn should be associated with a single form input. In this particular case, there is already a (very poorly named) form input that does something like this: GroupInput. I'm not even sure if it still works, and if we're going to "resurrect" it now that somebody has a use case for it (the original code in Fusion has since been removed), perhaps it should be rewritten, but essentially what GroupInput is is a form wrapped up into an input; when the "parent" form is submitted, the "child" form is also submitted, and the result of submitting the "child" form becomes the value of the GroupInput in the "parent" form.
Based on what Jonathan and I were able to reconstruct from memory about the IRC discussion around this issue, the actual use case here is a form that has multiple inputs, which map to a single Item attribute, a reference to another Item which has the values of those inputs set on it. Unfortunately the original bug description doesn't cover this at all; I've often found it very useful to be more specific about the concrete use case that motivates a more general change, even if the benefits are "obvious".
Anyhow, I think this is the wrong way of going about things. A single item attribute should be associated with a single model parameter, which in turn should be associated with a single form input. In this particular case, there is already a (very poorly named) form input that does something like this: GroupInput. I'm not even sure if it still works, and if we're going to "resurrect" it now that somebody has a use case for it (the original code in Fusion has since been removed), perhaps it should be rewritten, but essentially what GroupInput is is a form wrapped up into an input; when the "parent" form is submitted, the "child" form is also submitted, and the result of submitting the "child" form becomes the value of the GroupInput in the "parent" form.