Basically you would have a phx-change event on the form which will fire on every input change. Then in the handle_event callback you will pattern match on your Company select value and see if it changed from the one you have in your socket assigns. If it did, fetch the new Signs and re-assign. This will re-render your liveview automatically.

While there isn’t an example exactly like your use-case in it, phoenix_live_view_example is a good way to peek at how things work.

Later edit: found this (the “Bike comparison” example)

Thank you for the suggestion and example. I managed to populate the select field by using LiveView.

As suggested, I use phx-change that will pattern match the Company select value, then fetch the new Sign based on the value. Also assign the selected value to the Company select field.

This is part of my LiveView file.

  def render(assigns), do: LiveDeviceView.render("form.html", assigns)

  def mount(session, socket) do
    companies = Portal.Fleet.list_companies()
    selected_company = nil
    
    {:ok,
     assign(socket,
       changeset: Fleet.change_device(%Device{}),
       companies: companies,
       selected_company: selected_company,
       signs: [],
     )}
  end

  def handle_event("populate_sign", %{"device" => params}, socket) do
    signs = Fleet.get_company(String.to_integer(params["company_id"]))
    companies = Portal.Fleet.list_companies()
    changeset = Fleet.change_device(%Device{})
    selected_company = String.to_integer(params["company_id"])

    {:noreply, assign(socket, signs: signs, companies: companies, selected_company: selected_company, changeset: changeset)}
  end

form.html.leex

<%= f = form_for @changeset, "#", [phx_change: :populate_sign, phx_submit: :save] %>

  <%= label f, :company_id %>
  <%= select f, :company_id, Enum.map(@companies, &{&1.name, &1.id}), prompt: "Select a company", selected: @selected_company %>
  <%= error_tag f, :companies %>

  <%= if @signs == [] do %>
    <%= if @selected_company != nil do %> 
      <p>This company has no Sign</p> 
      <%= link "Add a new sign for this company", to: Routes.company_path(@socket, :new) %>
    <% end %>
  <% else %>
    <%= label f, :sign_id %>
    <%= select f, :sign_id, Enum.map(@signs, &{&1.name, &1.id}), prompt: "Select a sign" %>
    <%= error_tag f, :signs %>
  <% end %>

    <%= submit "Save", phx_disable_with: "Saving..." %>
  </div>

</form>

Thank you again for all responses.