Forms with nested lists that Rails will understand

[dhnaranjo]

Continuing from a conversation in #312, building a form that is compatible with Rails accepts_nested_attributes_for and ActionController::Parameters is a bit fussy. When you use Rails form helper fields_for to act on a list from a has_many relationship it generates field names like user[things_fields][0][name].

To duplicate this behavior with Phlex::Collection I was taking advantage of the @index ivar. @joeldrapper cut that for concerns that folks would, for example, misuse it to poorly replicate things you should do with CSS.

Made an executable test gist demonstrating the inputs ActionController::Parameters demands to give us the shape accepts_nested_attributes_for demands.

So we need a unique integer key. Alternatives to Phlex::Collection passing the index could be:

• Generating a timestamp
• Using the record ID, tho this is tricky if you use UUID keys, which I do
• I dunno. Something else?

Thoughts?