Symfony 2 constructor unit testing

I am quite lost with how I should test the constructor. I have basic knowledge of how test are suppose to work but in this case, I do not know how I should proceed.

I have a constructor that handles rest request.

    public function getStuffAction(Request $request) {
        $searchString = $request->query->get('q', '');

        if ($searchString === '')
            $stuff = $this->getManager('stuff')->findAll();
        else
            $stuff = $this->getManager('stuff')->findBySearchString($searchString);

        $view = View::create();
        $view->setData($stuff)->setStatusCode(200);
        return $view;
    }

    public function getManager($entityName) {
        return $this->get('app.' . $entityName . '.manager');
    }

I understand that I need to test the behaviour of my function. I see that I should test when the query is empty, when it is not and probably if it return something.

Be aware that I am new to Symfony.

From what I read, I should inject the manager as a parameter in getStuffAction so it can then be mocked but I don't know how.