Error when persisting doctrine entity with OneToMany association - symfony

Question

New entities in a collection using cascade persist will produce an Exception and rollback the flush() operation. The reason is that the "UserGroupPrivilege" entity has identity through a foreign entity "UserGroup".
But if the "UserGroupPrivilege" has its own identity with auto generated value the code works just fine, and I don't want that I want the identity to be a composite key to enforce validation. here is my code:
Entity UserGroup:
class UserGroup
{
/**
* #var integer
*
* #ORM\Column(type="integer", nullable=false)
* #ORM\Id
* #ORM\GeneratedValue(strategy="IDENTITY")
*/
private $id;
/**
* #var integer
*
* #ORM\Column(type="boolean", nullable=false)
* #Type("integer")
*/
private $active;
/**
* #ORM\OneToMany(targetEntity="UserGroupPrivilege", mappedBy="userGroup", cascade={"persist"})
*/
private $privileges;
Entity UserGroupPrivilege:
class UserGroupPrivilege
{
/**
* #var integer
*
* #ORM\Id
* #ORM\Column(type="integer", nullable=false)
*
*/
private $privilegeId;
/**
* #var UserGroup
*
* #ORM\Id
* #ORM\ManyToOne(targetEntity="UserGroup", inversedBy="privileges")
* #ORM\JoinColumn(name="userGroupId", referencedColumnName="id")
*/
private $userGroup;
/**
* #var string
* #ORM\Column(type="string", nullable=false)
*/
private $name;
/**
* #var string
* #ORM\Column(type="string", nullable=false)
*/
private $value;
Controller:
$userGroup = new UserGroup();
$userGroupPrivilege = new UserGroupPrivilege();
userGroupPrivilege->setUserGroup($userGroup)
->setName($arrPrivilege['name'])
->setValue($arrPrivilege['value'])
->setPrivilegeId($arrPrivilege['privilegeId']);
$userGroup->addPrivilege($userGroupPrivilege);
$data = $repo->saveUserGroup($userGroup);
return $data;
Repository:
$em = $this->getEntityManager();
$em->persist($userGroup);
$em->flush();
I get the following error:
Entity of type UserGroupPrivilege has identity through a foreign entity UserGroup, however this entity has no identity itself. You have to call EntityManager#persist() on the related entity and make sure that an identifier was generated before trying to persist 'UserGroupPrivilege'. In case of Post Insert ID Generation (such as MySQL Auto-Increment or PostgreSQL SERIAL) this means you have to call EntityManager#flush() between both persist operations.

Answer 1

Error message is pretty self explanatory. To relate UserGroupPrivilege to UserGroup, UserGroup must have it's ID set. However, since you've just created both entities it has no id because it hasn't been persisted to database yet.
In your case :
$em = $this->getEntityManager();
$em->persist($userGroup);
$em->persist($userGroupPrivilege);
$em->flush();

Answer 2

Can you "enforce validation" with unique constraint:
/**
* #Entity
* #Table(uniqueConstraints={#UniqueConstraint(name="ugppriv_idx", columns={"priviledgeId", "userGroup"})})
*/
class UserGroupPriviledge
{
...