cart_form.html.eex

<%= form_for @conn, "/cart", [class: "cart-form form-inline", as: :cart], fn f -> %>
  <div class="form-group">
    <div class="input-group">
      <%= hidden_input f, :product_name, value: @product.name %> 
      <%= hidden_input f, :pack_size, value: @product.pack_size %>
      <%= hidden_input f, :product_id, value: @product.id %>
      <div class="input-group-addon">Qty</div>
      <%= number_input f, :quantity, value: 1, class: "form-control", style: "width: 70px"  %>
    </div>
    <%= error_tag f, :quantity %>
  </div>

  <%= submit "Add to cart", class: "btn btn-primary pull-right" %>
<% end %>