public function testPrePersist()
 {
     $dateCreated = new \DateTime();
     $dateCreated = $dateCreated->format('yy');
     $this->assertNull($this->unit->getCreatedAt());
     $this->assertNull($this->unit->getUpdatedAt());
     $this->unit->prePersist();
     $this->assertEquals($dateCreated, $this->unit->getCreatedAt()->format('yy'));
     $this->assertEquals($dateCreated, $this->unit->getUpdatedAt()->format('yy'));
 }