After creating a model and adding it to a repository I want to have the new ID for different purposes (creating a mail, updating other fields outside the Extbase world)
$page = t3lib_div::makeInstance('Tx_MyExt_Domain_Model_Page');
$page->setTitle('Hello World');
$this->pageRepository->add($page);
At this point $page hasn't got an ID yet, uid is null.
$page->getUid(); // returns null
When does it get it? And how can I retrieve in on runtime?
In ExtBase, objects are "managed". This means every persistence transaction (add/remove/update) is simply noted in the underlying logic, but not yet executed until the appropriate time (like the end of processing a request). So, just because you add an object to a repository doesn't mean that it's actually added yet. That actually happens once $persistenceManager->persistAll() is called, which isn't something you need to do manually, ever. The point is, your $page object won't have a UID until it's saved and that's why $page->getUid() returns null. Look here for a great explanation.
I suspect that you are trying to do something outside of the ExtBase object/MVC lifecycle. At least, last time I got null when I tried to get the UID of an object, it was because I wasn't operating within the framework appropriately.
However, if you post some more code and give us a bigger picture of what you're trying to achieve, maybe we can help you get to a point where that object actually has a UID. For instance, if you're in a Controller object, tell us which Action method you're in, or if you're in a Repository object, tell us what you're trying to get from the repository and where/how you plan on using the query results.
EDIT
Just guessing here, but I'm assuming you're executing this code in some action of a controller. Since after the controller is executed a view is rendered, you can just pass the page object to the view:
$this->view->assign('page', $page);
And then in your view you can use the page object in a link:
<f:link.action action="show" arguments="{page:page}">
See this page object
</f:link.action>
And then in the show action of your controller you can show the page:
public function showAction(Tx_MyExt_Domain_Model_Page $page) {
// Do whatever you need to show the page in the `Show.html` template
}
I really am just guessing here. If you can give us a larger picture of what you're trying to do, what your action methods are supposed to do and things like that, we can answer your question a little more confidently.
(I'm also assuming that your page object isn't a replacement for the regular TYPO3 pages and that they are something totally different. It's much easier to deal with those TYPO3 pages through the backend interface than at the php level.)
You can call persistence manager explicitly in Your controller like this
#TYPO3 4.x
$persistenceManager = $this->objectManager->create('Tx_Extbase_Persistence_Manager');
$persistenceManager->persistAll();
#TYPO3 6.x
$persistenceManager = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\CMS\Extbase\Persistence\Generic\PersistenceManager');
$persistenceManager->persistAll();