public function getCustomers($criteria = null, ConnectionInterface $con = null)
    {
        if (null === $this->collCustomers || null !== $criteria) {
            if ($this->isNew() && null === $this->collCustomers) {
                // return empty collection
                $this->initCustomers();
            } else {
                $collCustomers = CustomerQuery::create(null, $criteria)
                    ->filterByCustomerGroup($this)
                    ->find($con);
                if (null !== $criteria) {
                    return $collCustomers;
                }
                $this->collCustomers = $collCustomers;
            }
        }

        return $this->collCustomers;
    }