Trying to delete ManyToMany relatrionship with composite keys.

    /**
     * @ORM\Id
     * @ORM\ManyToOne(targetEntity="Entity\User\Account", inversedBy="memberships")
     * @ORM\JoinColumn(name="uid", referencedColumnName="uid")
     */
    protected $userAccount;

    /**
     * @ORM\Id
     * @ORM\ManyToOne(targetEntity="Entity\Merchant\Account")
     * @ORM\JoinColumn(name="mch_accountid", referencedColumnName="accountid")
     */
    protected $merchantAccount;

    /**
     * @ORM\Column(type="datetime", name="date")
     * @var datetime
     */
    protected $date;

    /**
     * @ORM\Column(type="boolean")
     * @var boolean
     */
    protected $administrator;

    /**
     * @ORM\ManyToMany(targetEntity="Entity\User\Privilege", indexBy="privilegeid")
     * @ORM\JoinTable(name="fsbackend.user_mch_account_privilege",
     *   joinColumns={
     *       @ORM\JoinColumn(name="mch_accountid", referencedColumnName="mch_accountid"),
     *       @ORM\JoinColumn(name="uid", referencedColumnName="uid")
     *   },
     *   inverseJoinColumns={
     *       @ORM\JoinColumn(name="privilegeid", referencedColumnName="privilegeid")
     *   }
     * )
     */
    protected $privileges;

    $membership->getPrivileges()->clear();

    $this->_em->persist($userAccount);
    $this->_em->flush();