/**
* User must not subscribe lesson and not be the owner.
*
* @param User $user
* @param Lesson $lesson
* @return bool
*/
public function subscribe(User $user, Lesson $lesson) : bool
{
return Lesson::whereId($lesson->id)->where('lessons.owner_id', '!=', $user->id)->where('lessons.visibility', '=', 'public')->whereNotIn('lessons.id', $user->subscribedLessons()->pluck('id'))->exists();
}