MultiValueField does not render correctly with as_hidden

[Bob Thomas]

MultiValueField needs to define hidden_widget = MultipleHiddenInput so that it will work properly when rendered with as_hidden(). However, MultipleHiddenInput still doesn't render IDs the same way as MultiWidget does, so MultiWidget's value_from_datadict will not find the values that MultipleHiddenInput has rendered. Or something like that. I'm concerned that trying to fix MultipleHiddenInput to work with MultiValueField will break MultipleChoiceField, so fixing this may require another Widget class.

A concrete example for this is using form preview with a SplitDateTimeField. The preview page renders all fields with as_hidden(), which creates a field like

<input type="hidden" name="delay_start" value="[u&#39;2008-08-29&#39;, u&#39;00:00:00&#39;]" id="formtools_delay_start" />

...which can't be turned back into valid data. It needs to render multiple hidden fields like:

<input type="hidden" name="delay_start_0" value="2008-08-29" id="formtools_delay_start_0" />
<input type="hidden" name="delay_start_1" value="00:00:00" id="formtools_delay_start_1" />

Using MultipleHiddenInput creates 2 hidden fields, but does not append the _i to each name and ID, so the data still does not get read back correctly by the form.

[Bob Thomas]

A new widget for rendering a split input field as multiple hidden fields

[Bob Thomas]

Attached patch with a new SplitHiddenInput widget, which is an appropriate replacement for MultiWidget when a hidden field is needed. SplitDateTimeField now uses a class derived from it for its hidden_widget. Needs tests.

[Bob Thomas]

Looks like this was just fixed by [8816] for ticket #7975