public function addUser(ChildUser $user)
    {
        if ($this->collUsers === null) {
            $this->initUsers();
        }

        if (!$this->getUsers()->contains($user)) {
            // only add it if the **same** object is not already associated
            $this->collUsers->push($user);
            $this->doAddUser($user);
        }

        return $this;
    }