How to add entity manager in resolver

[ace90]

Q	A
Bug report?	yes/no
Feature request?	yes/no
BC Break report?	yes/no
RFC?	yes/no
Version/Branch	x.y.z

I'm new in graphql. I try to add entity manger in my resolver using injecting service in constructor

class Characters
{
	public $em;

	public function __construct(EntityManagerInterface $em)
	{
		$this->em = $em;
	}

      public function getHumans()
          {
              dd($this);
              $humans = self::findByType(self::TYPE_HUMAN);
      
              return $humans;
          }
}

but when i query this. I always get this error Using $this when not in object context. May i know how can I fix this issue?

in my service.yml. I try this one too

App\Resolver\Characters:
        arguments: ['@doctrine.orm.entity_manager']

[mcg-web]

You should declare your service as a resolver. You can see how this can be done here.