Cupon.php

class Cupon extends Model
{
  /**
   * Получить продукт где купон id связан.
   */
  public function product()
  {
    return $this->belongsTo('App\Product');
  }
}

foreach.php

$cupon = App\Cupon::find(1); - найти купон по id = 1

echo $cupon->product->title; 
____________________________
$product = App\Product::find(1);

foreach($product->cupons as $cupon) {
$cupon-id;
}
____________________________________
$prodcuts = App\Product::findAll();

foreach($products as $product) {
    foreach($product->cupons as $cupon) {
    $cupon->id
 }
}
___________________________________________
$cupons = App\Product::find(1)->cupons;
foreach($cupons as $cupon) 
{ $cupon->id }

Product.php

class Product extends Model
{
  /**
   * Получить купон.
   */
  public function cupons()
  {
    return $this->hasMany('App\Cupon');
  }
}

Спасибо)