"""Reusable queryset annotations for the Course API.

Centralised here so list + detail views always return the same shape, and
expensive aggregations are computed via correlated subqueries (single SQL
statement, no Cartesian explosion across multi-relation joins).
"""

from datetime import timedelta

from django.db.models import Count, IntegerField, OuterRef, Subquery, Sum
from django.db.models.functions import Coalesce
from django.utils import timezone


def _published_lesson_qs():
    from apps.courses.models import Lesson, LessonStatus
    return Lesson.objects.filter(status=LessonStatus.PUBLISHED)


def annotate_course_stats(qs, *, bestseller_window_days: int = 90):
    """Annotate Course queryset with `lesson_count`, `total_minutes_seconds`,
    `enrolled_count`, and `popularity` (= paid orders in the last
    `bestseller_window_days`). Each is a correlated subquery so they don't
    interact — safe to combine."""

    from apps.courses.models import Lesson, LessonStatus
    from apps.enrollments.models import Enrollment
    from apps.payments.models import Order, OrderStatus

    cutoff = timezone.now() - timedelta(days=bestseller_window_days)

    lesson_count_sq = (
        Lesson.objects.filter(section__course=OuterRef("pk"), status=LessonStatus.PUBLISHED)
        .order_by()
        .values("section__course")
        .annotate(c=Count("id"))
        .values("c")[:1]
    )
    duration_sum_sq = (
        Lesson.objects.filter(section__course=OuterRef("pk"), status=LessonStatus.PUBLISHED)
        .order_by()
        .values("section__course")
        .annotate(s=Coalesce(Sum("duration_seconds"), 0))
        .values("s")[:1]
    )
    enrolled_count_sq = (
        Enrollment.objects.filter(course=OuterRef("pk"))
        .order_by()
        .values("course")
        .annotate(c=Count("id"))
        .values("c")[:1]
    )
    popularity_sq = (
        Order.objects.filter(
            course=OuterRef("pk"),
            status=OrderStatus.PAID,
            created_at__gte=cutoff,
        )
        .order_by()
        .values("course")
        .annotate(c=Count("id"))
        .values("c")[:1]
    )

    return qs.annotate(
        lesson_count=Coalesce(Subquery(lesson_count_sq, output_field=IntegerField()), 0),
        total_minutes_seconds=Coalesce(Subquery(duration_sum_sq, output_field=IntegerField()), 0),
        enrolled_count=Coalesce(Subquery(enrolled_count_sq, output_field=IntegerField()), 0),
        popularity=Coalesce(Subquery(popularity_sq, output_field=IntegerField()), 0),
    )