Table Inputs

form.html.erb

<table>
  <thead>
    <tr>
      <th width="350px">Producto/Servicio</th>
      <th>Cantidad</th>
      <th>Precio</th>
      <th>Total</th>
    </tr>
  </thead>
  <tbody>
    <%= form.fields_for :output_items , id: "multiplicar" do |output_item| %>
      <tr>
        <td><%= output_item.collection_select :item_id, Item.all, :id, :name, { :include_blank => "Seleccionar item" } %></td>
        <td><%= output_item.number_field :quantity, class: "quantity", onChange: "multiplicar();" %></td>
        <td><%= output_item.number_field :price, class: "price", onChange: "multiplicar();" %></td>
        <td><input type="text" disabled="true" id="total" ></td>
      </tr>
    <% end %>            
  </tbody>
</table>

<script>

    function multiplicar(){

      input1 = document.querySelector(".quantity").value;
      input2 = document.querySelector(".price").value;
      total = parseInt(input1)*parseInt(input2);
      document.getElementById("total").value = total;

}

</script>