_form.rb

= simple_form_for @machine_visit_form do |f|
  = f.error_notification

  = f.input :refill_id, as: :hidden
  = f.input :refill_date, as: :date
  = f.input :refill_counter, as: :decimal, input_html: {value: number_with_precision(f.object.refill_counter, precision: 2) }

  - @machine_visit_form.refill_line_items.each_with_index do |rli, i|
    .well
      = "#{rli.product_name}"
      = f.fields_for rli do |ff|
        = ff.hidden_field :id
        = ff.select :product, options_from_collection_for_select(Product.all, "id", "name")
        = ff.number_field :quantity

source.html

          <div class="well">
            No Product<input id="refill_refill_line_item_id" name=
            "refill[refill_line_item][id]" type="hidden" /><select id=
            "refill_refill_line_item_product" name="refill[refill_line_item][product]">
              <option value="1">
                Mars Bar
              </option>
            </select><input id="refill_refill_line_item_quantity" name=
            "refill[refill_line_item][quantity]" type="number" />
          </div>

          <div class="well">
            No Product<input id="refill_refill_line_item_id" name=
            "refill[refill_line_item][id]" type="hidden" /><select id=
            "refill_refill_line_item_product" name="refill[refill_line_item][product]">
              <option value="1">
                Mars Bar
              </option>
            </select><input id="refill_refill_line_item_quantity" name=
            "refill[refill_line_item][quantity]" type="number" />
          </div>

          <div class="well">
            No Product<input id="refill_refill_line_item_id" name=
            "refill[refill_line_item][id]" type="hidden" /><select id=
            "refill_refill_line_item_product" name="refill[refill_line_item][product]">
              <option value="1">
                Mars Bar
              </option>
            </select><input id="refill_refill_line_item_quantity" name=
            "refill[refill_line_item][quantity]" type="number" />
          </div><input class="btn btn-primary" name="commit" type="submit" value=
          "Create Refill" />