Ubercart: how to limit the number of products of a type in the cart

The solution I proposed was to implement two hooks in a small custom module: hook_uc_add_to_cart (to prevent the user from adding products with the Add to cart button), and hook_form_uc_cart_view_form_alter (to prevent the user from modifying the quantity of a product through the cart form).

This sample code below hard-codes the "3" limit and only for product with nids 7 and 8, so, ideally, you'd create a config file and admin ui to enter the maximum, and also to select the target nid of the product. Or you can create a "product feature".  

You also need to create a .info file for this, and save as uc_restrict_by_nid.info

So, this is only a STARTING POINT for a better solution.  

/**
 * Implements hook_uc_add_to_cart().
 */
function uc_restrict_by_nid_uc_add_to_cart($nid, $qty, $data) {
  $qty = 3;
  $target_nids = array(7, 8); // Enter the NIDs of the restricted products here. 

  // Skip this if added product is not included in the target nids.
  if (!in_array($nid, $target_nids)) {
    return;
  }

  $cart_items = uc_cart_get_contents();
  $count = 1;  // Count the item being added.
  foreach ($cart_items as $cart_item) {
    if ($cart_item->nid == $nid) {
      $count += $cart_item->qty;
    }
  }

  if ($count > $qty) {
    $product = node_load($nid);
    $result[] = array(
      'success' => FALSE,
      'message' => t('You may not have more than @qty %product to your cart.', array('@qty' => $qty, '%product' => $product->title)),
    );
    return $result;
  } 
}

/** 
 * Implements hook_form_FORM_ID_alter().
 */
function uc_restrict_by_nid_form_uc_cart_view_form_alter(&$form, &$form_state) {
  // Insert custom validation callback.
  $form['#validate'][] = 'uc_restrict_by_nid_limit_products_in_cart'; 
}

/** 
 * Custom validation callback.
 */
function uc_restrict_by_nid_limit_products_in_cart($form, $form_state) {
  $target_nids = array(7, 8);
  $qty = 3;

  $items = $form_state['values']['items'];
  $item_counts = array();
  foreach ($items as $item) {
    if (in_array($item['nid'], $target_nids)) {
      if (!isset($item_counts[$item['nid']])) { 
        $item_counts[$item['nid']] = 0;
      }
      $item_counts[$item['nid']] += $item['qty']; 
    }
  }
  foreach ($item_counts as $nid => $item_count)  {
    if ($item_count > $qty) {
      $product = node_load($nid);
      form_set_error('form', t('You may not have more than @qty items of product %product', array('@qty' => $qty, '%product' => $product->title)));
    }
  }
}