"""One Watch later list per user, held by the database

Revision ID: 0112_one_watch_later
Revises: 0111_audit_request_id
Create Date: 2026-10-20

Watch later was made on a user's first save by a read then an insert, so two first saves at once
each made a list, and the reader then picked between them at random. A partial unique index holds
"one per user" now. An instance that already has a second list keeps the oldest: the others' videos
move into it (after its own, in their order; one it already holds is not added twice), then they go.
"""

import sqlalchemy as sa
from alembic import op

revision: str = "0112_one_watch_later"
down_revision: str | None = "0111_audit_request_id"
branch_labels: str | None = None
depends_on: str | None = None

_EXTRA = """
    SELECT p.id, k.keep_id
    FROM playlists p
    JOIN (
        SELECT user_id, min(id) AS keep_id FROM playlists WHERE kind = 'watch_later' GROUP BY user_id
    ) k ON k.user_id = p.user_id
    WHERE p.kind = 'watch_later' AND p.id <> k.keep_id
"""


def upgrade() -> None:
    op.execute(
        f"""
        INSERT INTO playlist_items (playlist_id, video_id, position, added_at)
        SELECT e.keep_id, i.video_id,
               (SELECT coalesce(max(position), -1) FROM playlist_items WHERE playlist_id = e.keep_id)
               + row_number() OVER (PARTITION BY e.keep_id ORDER BY i.playlist_id, i.position, i.id),
               i.added_at
        FROM playlist_items i
        JOIN ({_EXTRA}) e ON e.id = i.playlist_id
        ON CONFLICT ON CONSTRAINT uq_playlist_video DO NOTHING
        """
    )
    op.execute(f"DELETE FROM playlists WHERE id IN (SELECT id FROM ({_EXTRA}) e)")
    op.create_index(
        "uq_playlist_watch_later",
        "playlists",
        ["user_id"],
        unique=True,
        postgresql_where=sa.text("kind = 'watch_later'"),
    )


def downgrade() -> None:
    op.drop_index("uq_playlist_watch_later", table_name="playlists")