/**
  * Generates an attribute's key on base of its values
  *
  * @param ProductAttributeInterface $productAttribute
  *
  * @return string
  */
 protected function generateProductAttributeKey(ProductAttributeInterface $productAttribute)
 {
     $values = [];
     $productAttribute->getAttributeValues()->map(function (AttributeValueInterface $attributeValue) use(&$values) {
         $values[$attributeValue->getId()] = $attributeValue->getId();
     });
     ksort($values);
     return implode(',', $values);
 }