How to unit test getId() method on Symfony2 ORM entity?

I want to unit test all my Catalog Entity methods and have a problem with testing getter getId(). When I create the Catalog (new Catalog()) there is no Id set and no setId() method!

My first stupid step was to write the mok:

public function testGetId()
{
    $category = $this->getMock('\Dimas\CatalogBundle\Entity\Category');
    $category->expects($this->any())
        ->method('getId')
        ->will($this->returnValue(352));
    $this->assertEquals(352, $category->getId());
}

But this dummy-test did not tested my entity!

What is the right way to test getId() method?