test that use case throws error if non existant user id given

app/Plan/UseCases/CreatePlan.php

@@ -6,6 +6,7 @@ use App\Plan\CreatePlanDto;
 use App\Plan\Plan;
 use App\Plan\PlanRepository;
 use App\User\UserRepository;
+use DomainException;
 
 class CreatePlan
 {
@@ -14,9 +15,17 @@ class CreatePlan
         private UserRepository $userRepo,
     ) {}
 
+    /**
+     * @throws DomainException
+     */
     public function execute(CreatePlanRequest $request): Plan
     {
-        $user = $this->userRepo->find($request->userId);
+        $userId = $request->userId;
+        $user = $this->userRepo->find($userId);
+
+        if ($user === null) {
+            throw new DomainException("User with id: $userId doesnt exist");
+        }
 
         return $this->planRepo->create(new CreatePlanDto(
             name: $request->name,

tests/Unit/Plan/UseCases/CreatePlanTest.php

@@ -8,6 +8,7 @@ use App\User\UseCases\CreateUserDto;
 use App\User\User;
 use App\User\UserRepository;
 use App\ValueObjects\EmailAddress;
+use DomainException;
 use Tests\Fakes\FakePlanRepository;
 use PHPUnit\Framework\TestCase;
 use Tests\Fakes\FakeUserRepository;
@@ -48,4 +49,14 @@ class CreatePlanTest extends TestCase
         ));
         $this->assertInstanceOf(User::class, $plan->getUser());
     }
+
+    public function test_nonexistant_user_id_throws(): void
+    {
+        $this->expectException(DomainException::class);
+        $this->expectExceptionMessage("User with id: 1 doesnt exist");
+        $plan = $this->useCase->execute(new CreatePlanRequest(
+            userId: 1,
+            name: 'testPlan',
+        ));
+    }
 }