public function addCustomer(ChildCustomer $customer)
    {
        if ($this->collCustomers === null) {
            $this->initCustomers();
        }

        if (!$this->collCustomers->contains($customer)) { // only add it if the **same** object is not already associated
            $this->doAddCustomer($customer);
            $this->collCustomers[] = $customer;
        }

        return $this;
    }