"""phase16 — ensure digest_pending and inspection_id columns on notifications

Background
----------
The `notifications` table was created before the Alembic migration chain was
established (pre-phase1 baseline schema).  Two columns added to the model
after the initial creation were never covered by a migration:

  digest_pending  BOOLEAN NOT NULL DEFAULT 0  (used by the digest email system)
  inspection_id   INT NULL FK → inspections.id ON DELETE CASCADE

Without this migration, any instance whose `notifications` table was created
from the original baseline (rather than from the current model) will raise
`OperationalError: Unknown column 'notifications.digest_pending'` the first
time a notification is created, and the digest email cron will fail entirely.

All checks use INFORMATION_SCHEMA so the migration is safe to re-run on any
MySQL version ≥ 5.7 (CLAUDE.md rules 16, 17).

Revision ID: phase16_notifications_columns
Revises:     phase15_audit_log_indexes
"""

revision      = 'phase16_notifications_columns'
down_revision = 'phase15_audit_log_indexes'
branch_labels = None
depends_on    = None

from alembic import op
import sqlalchemy as sa


def _column_exists(conn, table, column):
    result = conn.execute(sa.text(
        "SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS "
        "WHERE TABLE_SCHEMA = DATABASE() "
        "AND TABLE_NAME = :table AND COLUMN_NAME = :col"
    ), {"table": table, "col": column})
    return result.scalar() > 0


def _index_exists(conn, table, index_name):
    result = conn.execute(sa.text(
        "SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS "
        "WHERE TABLE_SCHEMA = DATABASE() "
        "AND TABLE_NAME = :table AND INDEX_NAME = :idx"
    ), {"table": table, "idx": index_name})
    return result.scalar() > 0


def upgrade():
    bind = op.get_bind()

    # 1. digest_pending — Boolean NOT NULL DEFAULT 0
    #    Used by notify() to flag notifications for digest delivery, and by
    #    send_pending_digests() to find and clear them after delivery.
    if not _column_exists(bind, 'notifications', 'digest_pending'):
        op.execute(sa.text(
            "ALTER TABLE notifications "
            "ADD COLUMN digest_pending TINYINT(1) NOT NULL DEFAULT 0"
        ))

    # 2. Index on digest_pending — the digest cron filters on this column
    if not _index_exists(bind, 'notifications', 'ix_notifications_digest_pending'):
        op.execute(sa.text(
            "CREATE INDEX ix_notifications_digest_pending "
            "ON notifications (digest_pending)"
        ))

    # 3. inspection_id — nullable FK to inspections, CASCADE on delete
    #    Allows the notification bell to link directly to an inspection.
    if not _column_exists(bind, 'notifications', 'inspection_id'):
        op.execute(sa.text(
            "ALTER TABLE notifications "
            "ADD COLUMN inspection_id INT NULL, "
            "ADD CONSTRAINT fk_notifications_inspection_id "
            "    FOREIGN KEY (inspection_id) REFERENCES inspections(id) "
            "    ON DELETE CASCADE"
        ))


def downgrade():
    bind = op.get_bind()

    if _column_exists(bind, 'notifications', 'inspection_id'):
        op.execute(sa.text(
            "ALTER TABLE notifications "
            "DROP FOREIGN KEY fk_notifications_inspection_id, "
            "DROP COLUMN inspection_id"
        ))

    if _index_exists(bind, 'notifications', 'ix_notifications_digest_pending'):
        op.execute(sa.text(
            "DROP INDEX ix_notifications_digest_pending ON notifications"
        ))

    if _column_exists(bind, 'notifications', 'digest_pending'):
        op.execute(sa.text(
            "ALTER TABLE notifications DROP COLUMN digest_pending"
        ))