Hi!
I would like to ask for help. I have a form and I would like to change the values of some fields if I change one of them.
For example, I have this code snippet:

@impl true
  def handle_event("validate", %{"_target" => ["invoice", "date"], "invoice" => invoice_params}, socket) do
    %{"date" => date} = invoice_params
    invoice = %{
      socket.assigns.invoice
      | due_date: Date.add(Date.from_iso8601!(date), socket.assigns.invoice.payment_term),
        date: Date.from_iso8601!(date)
    }

    changeset =
      invoice
      |> Invoices.change_invoice(invoice_params)
      |> Ecto.Changeset.put_assoc(:invoice_lines, invoice.invoice_lines)
      |> Map.put(:action, :validate)

    {:noreply,
     socket
     |> assign(:invoice, invoice)
     |> assign(:changeset, changeset)}
  end

I want to update the due_date field. But this does not change the field in the form.
I’ve manually updated the socket.assigns.invoice, it doesn’t work either.

I have a partner field which works though. When I select a partner, it goes through a few updates. The problem there is that the validation on the other field (date) does not work. Here is the code for that:

@impl true
 def handle_event(
     "validate",
     %{"_target" => ["invoice", "partner_id"], "invoice" => %{"partner_id" => partner_id}} = invoice_params,
     socket) when partner_id != "" do
   partner = Partners.get_partner!(partner_id) |> Repo.preload([:bank_accounts])
   invoice = %{
     socket.assigns.invoice
     | partner_id: partner.id,
       payment_method: partner.payment_method,
       payment_term: partner.payment_term,
       currency_id: partner.currency_id,
       due_date: Date.add(socket.assigns.invoice.date, partner.payment_term),
       exchange_rate: exchange_rate(partner.currency_id)
   }
   changeset = invoice_changeset(invoice, invoice_params) |> Map.put(:action, :validate)

   {:noreply,
    socket
    |> assign(:partner, partner)
    |> assign(:invoice, invoice)
    |> assign(:partner, partner)
    |> assign(:changeset, changeset)
    |> assign(:bank_accounts, bank_accounts(partner.bank_accounts))}
 end

Is there anything in the documentation about this, or perhaps an example? I have not found anything similar here on the forum. Thank you!