one-to-one relations mapping

hello, im new here i dont know if this is the correct place, but try finding help on the web whitout success. so here is my issue.

im trying to map a one to one relation on 2 entities using the id field on both entities.

for example:

i have a customer entity and a cart entity, both entities have only an id field. and there is a FK from cart id to customer id so i make the mappings as follow:

first the customer:

Acme\DemoBundle\Entity\Customer:
type: entity
table: null
oneToOne:
cart:
targetEntity: Cart
mappedBy: customer
fields:
id:
type: integer
id: true
generator:
strategy: AUTO
lifecycleCallbacks: { }

now the cart:

Acme\DemoBundle\Entity\Cart:
type: entity
table: null
oneToOne:
customer:
targetEntity: Customer
inversedBy: cart
joinColumn:
name: id
referencedColumnName: id
fields:
id:
type: integer
id: true

lifecycleCallbacks: { }

the entities get generated ok, but im not able to insert in the cart entity:

im getting the followin exception:

Entity of type Acme\DemoBundle\Entity\Cart is missing an assigned ID for field 'id'. The identifier generation strategy for this entity requires the ID field to be populated before EntityManager#persist() is called. If you want automatically generated identifiers instead you need to adjust the metadata mapping accordingly.

when i made the insert a try this two ways:

$cart = new \Acme\DemoBundle\Entity\Cart();
$cart->setId($cart->getId());
$em->persist($cart);
$em->flush();

or this one

$cart = new \Acme\DemoBundle\Entity\Cart();
$cart->setCustomer($customer);
$em->persist($cart);
$em->flush();
in both cases i get the same exception

this is the symfony's log where the insert is generated by doctrine to insert the cart record

[2012-07-16 20:59:06] doctrine.DEBUG: SET NAMES UTF8 ([]) [] []
[2012-07-16 20:59:06] doctrine.DEBUG: INSERT INTO Customer (id) VALUES (null) ([]) [] []
[2012-07-16 20:59:06] event.DEBUG: Notified event "kernel.exception" to listener "Symfony\Component\HttpKernel\EventListener\ProfilerListener::onKernelException". [] []
[2012-07-16 20:59:06] event.DEBUG: Notified event "kernel.exception" to listener "Symfony\Component\HttpKernel\EventListener\ExceptionListener::onKernelException". [] []
[2012-07-16 20:59:06] request.CRITICAL: Doctrine\ORM\ORMException: Entity of type Acme\DemoBundle\Entity\Cart is missing an assigned ID for field 'id'. The identifier generation strategy for this entity requires the ID field to be populated before EntityManager#persist() is called. If you want automatically generated identifiers instead you need to adjust the metadata mapping accordingly. (uncaught exception) at /home/javier/intraway/doctrine_test/sf/vendor/doctrine/orm/lib/Doctrine/ORM/ORMException.php line 51 [] []

the only way i get this working is setting the generation strategy to auto but i dont thinks its ok, i want this two entities have the same id and if i make both use generators i cant be surethe ids get generated correctly.

thanks.