Rendering array field as hidden inputs

[Mathias Polligkeit]

I'm missing a way to automatically render an array field as hidden inputs. What I'd like to be able to do is this:

<%= form_for :some_form, "/", [hidden: [some_field: ["one", "two", "three"]]], fn f -> %>
  <%= hidden_inputs_for f %>
<% end %>

Which should render to:

<form action="/" method="post">
  <input type="hidden" id="something_0" name="something[]" value="one" />
  <input type="hidden" id="something_1" name="something[]" value="two" />
  <input type="hidden" id="something_2" name="something[]" value="three" />
</form>

Would it make sense to modify hidden_inputs_for/1 to behave that way? If so, I can open a PR.

Background: I'm currently working on tighter integration between flop, flop_phoenix and Phoenix.HTML.Form by implementing Phoenix.HTML.FormData for a struct. The goal is to make setting up filter forms based on the library structs easier. To work well with the sortable table and pagination components, parameters like the page size etc. are added to the :hidden field in form_for/2, so that all of those parameters can be rendered with a single call to hidden_inputs_for/1. For sorting however, an :order_by field is used, which is a list. I tried modifying the protocol implementation to include that list as hidden fields as well, but I wasn't successful, which means at the moment I would have to add a modified hidden_inputs_for/1 function to the my library. I'd rather have this part of Phoenix.HTML.

[José Valim]

Yeah, a PR is welcome.