protected function checkLessonMigrationStats()
    {
        $numberOfOriginalLessons = Lesson::activeOn($this->period)->count();
        $numberOfTrackedLessons = LessonMigration::all()->count();
        $numberOfMigratedLessons = ScoolLesson::all()->count();
        $this->output->info(' Original lessons: ' . $numberOfOriginalLessons);
        $this->output->info(' Tracked migrated lessons (table lesson_migration): ' . $numberOfTrackedLessons );
        $this->output->info(' Already migrated lessons: ' . $numberOfMigratedLessons);
        if ($numberOfTrackedLessons == 0 && $numberOfMigratedLessons == 0) return 0;
        if ( $numberOfOriginalLessons != $numberOfTrackedLessons ||
             $numberOfTrackedLessons != $numberOfMigratedLessons) return 1;
        if ( $numberOfOriginalLessons == $numberOfTrackedLessons ||
            $numberOfTrackedLessons == $numberOfMigratedLessons) return 2;
    }