Can't figure out how to use field_name/2 for sparse fieldsets

[kbaird]

I've been reading the docs, and found You can define custom fields or override current fields by defining inside the view function field_name/2 that takes data and conn as arguments. at https://hexdocs.pm/jsonapi/JSONAPI.View.html#module-fields

I've tried defining field_name/2 on my pertinent view module, but it seems to be ignored when fields/0 is present, and fields/0 being absent produces the error from https://github.com/jeregrine/jsonapi/blob/master/lib/jsonapi/view.ex#L138.

Is there any guidance I'm missing on how to comply with https://jsonapi.org/format/#fetching-sparse-fieldsets?

[kbaird]

Oh, I see. field_name is a meta-variable. So if you have a field called foobar, you need to define def foobar(data, conn), do: ... in your view module.

That said, I still don't see how to truncate the list of fields based on the request. I don't want nil (or some otherwise modified value) for a given field name - I want the field name to be absent entirely, which seems to be what the spec prescribes. And fields/0 not taking in the conn seems to make it an all-or-nothing affair.

[jherdman]

@jeregrine is this your intention in #120?