public function add(ActivityUsage $usage)
 {
     $key = $usage->getSubject();
     if (empty($this->usage[$key])) {
         $this->usage[$key] = array();
     }
     $this->usage[$key][] = $usage;
 }
 public function testGetSubject()
 {
     $this->assertEquals('created_from', $this->activityUsage->getSubject());
 }