How to add data when dynamically adding inputs using inputs_for + sort_param/drop_param?

I’m trying to allow a user to add tags to a parent using a has_many relationship. I’d like to offer ‘suggested’ tags that they can click and these are added to the form data. These are simple, single, text-entry forms.

I have a dynamic form using inputs_for with a button to add new, blank, form entries and remove existing values, per the standard Liveview documentation. I can add new tags, which is a single input, and this works as expected.

How can I programatically add new inputs_for entries with data in the form entry?

Can I use the value=“new” to set a value I can capture instead?

I’ve tried putting suggested tags as buttons with phx-click and handlers to capture the suggested tag value and put them in the parent struct as a struct item rather than param map, but I can’t work out how to have these render in the inputs_for in the same way as the ‘new’ tags.

eg, using the example code:

<.inputs_for :let={ef} field={@form[:emails]}>
  <input type="hidden" name="mailing_list[emails_sort][]" value={ef.index} />
  <.input type="text" field={ef[:email]} placeholder="email" />
  <.input type="text" field={ef[:name]} placeholder="name" />
    <.icon name="hero-x-mark" class="w-6 h-6 relative top-2" />

<input type="hidden" name="mailing_list[emails_drop][]" />

<button type="button" name="mailing_list[emails_sort][]" value="new" phx-click={JS.dispatch("change")}>
  add more

How could I have a button that would append an ef form item with an email value of ‘’?

sort_param doesn’t support such a usecase. It works from a mapping on a single form key to indexes, so there’s no way to pass any additional data.

The approach taken in One-to-Many LiveView Form | Benjamin Milde might work for you.

1 Like

That link was great and well written. Thanks! Now all working.

The trick in the link above to get the form data and update the changeset in a handler that doesn’t receive the form params, using get_assoc/put_assoc, worked great. I can keep all the existing code & behaviour the same, so a neat bonus is still getting deletion of dynamic inputs for ‘free’ from drop_param.

So to programatically add new inputs with data, you just need a button with a value and one additional handler:

 def handle_event("add_tag", %{"value" => tag}, socket) do
    new_tag = %SkillTag{
      tag: tag

    socket =
      update(socket, :form, fn %{source: changeset} ->
        existing = Ecto.Changeset.get_assoc(changeset, :skill_tags)
        changeset = Ecto.Changeset.put_assoc(changeset, :skill_tags, [new_tag | existing])

    {:noreply, socket}