public static function delete_unused()
{
$query = DB::connection()->prepare("SELECT * FROM Ingredient");
$query->execute();
$rows = $query->fetchAll();
foreach ($rows as $row) {
$ingredient = new Ingredient(array('ingredient_id' => $row['id'], 'name' => $row['name']));
if (is_null($ingredient->find_recipes_for_ingredient())) {
$ingredient->delete();
}
}
}