[TYPO3-mvc] Dependeny Injection in Model class not working?

[Matthias Krappitz]

Hi,

in one project I need to dynamically query something using a repository in a 
model class (I need a getter for the subcatgories of a category). Is 
dependency injection as it is working in controller classes also thought for 
model classes in this case? At least this code, that works in my controller 
class is not working in one of my model classes, $this->categoryRepository 
stays NULL:

	/**
	 * categoryRepository
	 *
	 * @var Tx_Travel_Domain_Repository_CategoryRepository
	 */
	protected $categoryRepository;

	/**
	 * injectCategoryRepository
	 *
	 * @param Tx_Tavel_Domain_Repository_CategoryRepository $categoryRepository
	 * @return void
	 */
	public function 
injectCategoryRepository(Tx_Dedontravel_Domain_Repository_CategoryRepository 
$categoryRepository) {
		$this->categoryRepository = $categoryRepository;
	}

Is there anyhing I need to do for this to work or is this just not 
implemented and I need to do it another way? If yes, how? 
t3lib_div::makeInstance()? Object Manager?

Another question concerning this to broaden my general understanding on 
this - in the controller classes is dependency injection like shown above 
THE way to go for all occasions of requiring a instanciated object? Or when 
should I still use $this->objectManager->... or is this deprecated now? I 
suppose t3lib_div::makeInstance() and new somehting (); is both absolutely 
never not meant to be used any more.

Matthias Krappitz
w. www.aemka.de