from django.core.management.base import BaseCommand
from django.db import transaction
from apps.course.models import Course, CourseChapter

class Command(BaseCommand):
    help = 'Migrates existing CourseLessons to be nested inside default CourseChapters (V2 Architecture).'

    def handle(self, *args, **options):
        self.stdout.write(self.style.WARNING("Starting data migration for Course Chapters..."))

        courses = Course.objects.all()
        courses_updated = 0
        lessons_migrated = 0

        # We wrap the whole loop in an atomic transaction. 
        # If anything crashes halfway through, the database rolls back to its original state!
        with transaction.atomic():
            for course in courses:
                # Check if course has lessons but no chapters yet
                if course.lessons.exists() and not course.chapters.exists():
                    self.stdout.write(f"Migrating course: {course.title}")
                    
                    # Create a default 'General' chapter
                    default_chapter = CourseChapter.objects.create(
                        course=course,
                        title="General",
                        priority=1
                    )
                    
                    # Attach all existing lessons to this new chapter
                    for index, course_lesson in enumerate(course.lessons.all().order_by('priority')):
                        course_lesson.chapter = default_chapter
                        course_lesson.priority = index + 1
                        course_lesson.save()
                        lessons_migrated += 1
                    
                    courses_updated += 1

        self.stdout.write(
            self.style.SUCCESS(
                f"🎉 Successfully migrated {lessons_migrated} lessons across {courses_updated} courses!"
            )
        )