• I suppose link and unlinked entities would then handled by UnitOfwork commit also?
• Since the collection is not initialized, one does not know upfront if the action will be successful, what happens if:
  • an entity is linked with a collection, although they are already connected.
  • an entity is unlinked from a collection it is not in.

Regarding the naming, i like link/unlink.

As far as usage is concerned, I currently imagine it as follows:

// EntityManager#link($sourceObj, $field, $targetObj)
$user = $em->getReference($userId); // $userId probably from request parameters
$address = $em->getReference($addressId); // $addressId probably from request parameters
$em->link($user, 'addresses', $address);

"What happens if: an entity is linked with a collection, although they are already connected."

Probably an SQL error which results in an exception from the driver. Depends on the database constraints though.

"What happens if: an entity is unlinked from a collection it is not in"

Probably nothing, at least not from the SQL side. An exception could be thrown from Doctrine itself if the update affected 0 rows.

Thanks for these initial questions. Thats definitely food for thought. Keep it coming.