Flarum\Core\Repository\DiscussionRepository::findOrFail PHP Method

    public function findOrFail($id, User $user = null)
    {
        $query = Discussion::where('id', $id);
        return $this->scopeVisibleTo($query, $user)->firstOrFail();
    }

 /**
  * @param PostReply $command
  * @return CommentPost
  * @throws \Flarum\Core\Exception\PermissionDeniedException
  */
 public function handle(PostReply $command)
 {
     $actor = $command->actor;
     // Make sure the user has permission to reply to this discussion. First,
     // make sure the discussion exists and that the user has permission to
     // view it; if not, fail with a ModelNotFound exception so we don't give
     // away the existence of the discussion. If the user is allowed to view
     // it, check if they have permission to reply.
     $discussion = $this->discussions->findOrFail($command->discussionId, $actor);
     $this->assertCan($actor, 'reply', $discussion);
     // Create a new Post entity, persist it, and dispatch domain events.
     // Before persistence, though, fire an event to give plugins an
     // opportunity to alter the post entity based on data in the command.
     $post = CommentPost::reply($discussion->id, array_get($command->data, 'attributes.content'), $actor->id, $command->ipAddress);
     if ($actor->isAdmin() && ($time = array_get($command->data, 'attributes.time'))) {
         $post->time = new DateTime($time);
     }
     $this->events->fire(new PostWillBeSaved($post, $actor, $command->data));
     $this->validator->assertValid($post->getAttributes());
     $post->save();
     $this->notifications->onePerUser(function () use($post, $actor) {
         $this->dispatchEventsFor($post, $actor);
     });
     return $post;
 }