Doctrine #id null when persisted - symfony

Question

The following works and a row in inserted into both tables:
$user = new User();
$user->setId(8484);
$user->setData("user test data");
$profile = new Profile();
$profile->setBlah(8484);
$profile->setData("profile test data");
// if I leave this out it works...
$user->setProfile($profile);
$em = $this->getDoctrine()->getEntityManager();
$em->persist($user);
$em->flush();
But if leave out $user->setProfile($profile); I get an error because User's id is null:
An exception occurred while executing 'INSERT INTO User (id, data) VALUES (?, ?)' with params {"1":null,"2":"user test data"}
How can it be?
class User
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
*/
protected $id;
/**
* #ORM\Column(type="string", length=64)
*/
protected $data;
/**
* #ORM\OneToOne(targetEntity="Profile", cascade={"persist", "remove"})
* #ORM\JoinColumn(name="id", referencedColumnName="blah")
*/
protected $profile;
}
class Profile
{
/**
* #ORM\Id
* #ORM\Column(name="blah", type="integer")
*/
protected $blah;
/**
* #ORM\Column(type="string", length=64)
*/
protected $data;
}
Set Profile method:
/**
* Set Profile
*
* #param \Test\AdminBundle\Entity\Profile $profile
* #return User
*/
public function setProfile(\Test\AdminBundle\Entity\Profile $profile = null)
{
$this->profile = $profile;
return $this;
}
EDIT:
If I change the joinColumn name to something random my object looks right using a var_dump but the query fails:
/**
* #ORM\OneToOne(targetEntity="Profile", cascade={"persist"})
* #ORM\JoinColumn(name="random_test", referencedColumnName="blah")
*/
Gives:
An exception occurred while executing 'INSERT INTO User (id, data, random_test) VALUES (?, ?, ?)' with params {"1":8484,"2":"user test data","3":null}:

Answer 1

You need to have your $profile persisted before Doctrine can successfully make a relationship between your user and your profile.
$em = $this->getDoctrine()->getEntityManager();
$user = new User();
$user->setId(8484);
$user->setData("user test data");
$profile = new Profile();
$profile->setBlah(8484);
$profile->setData("profile test data");
$em->persist($profile);
$user->setProfile($profile);
$em->persist($user);
$em->flush();
Think about what you are trying to do in your code in MySQL.
Originally you were saying:
Insert a user who's profile will be (8484)
Which results in (Err: Profile 8484 doesn't exist).
What you want to say is:
Insert a profile (8484).
Insert a user who's profile is (8484).