"""phase48 — scheduled follow-up: link a schedule back to its parent inspection

Ports single-tenant phase45 onto MT's `inspection_schedules` table. Adds:

    parent_inspection_id INT NULL   -- inspection this schedule is a follow-up of

Lets a follow-up be *planned for a later date* rather than started immediately —
the deferred twin of "Re-inspect Now". A one-time schedule carrying this column
is created via POST /api/v1/scheduled-inspections/follow-up; when the inspector
eventually starts it, the resulting inspection inherits `parent_inspection_id`,
so it lands as a true linked re-inspection: pre-filled from the parent, and
clearing the parent's `follow_up_required` on submit.

Without the column the scheduled run would be an ordinary inspection — no link,
no prefill, and the parent's follow-up flag would stay set forever.

Depends on phase45's `once` frequency: a follow-up is a single planned visit,
not a recurrence.

NULL means "not a follow-up", which is what every existing row is, so there is
no backfill and no schedule changes behaviour on deploy.

ON DELETE SET NULL: deleting the parent inspection must not cascade away a
schedule the inspector still has to perform — it just stops being a follow-up.
This mirrors `inspections.inspection_schedule_id`, which points the other way
with the same rule, so neither side of the pair can delete the other's history.

Uses INFORMATION_SCHEMA existence checks — safe to re-run on every tenant DB.
Additive only: nothing is renamed, retyped or dropped.

Revision id note
----------------
`alembic_version.version_num` is VARCHAR(32). The descriptive form
'phase48_schedule_parent_inspection' is 34 characters and fails the stamp UPDATE
after the DDL has already run. The id is abbreviated to fit, matching the
single-tenant chain's own 'phase45_sched_parent_insp'. The FILENAME stays
descriptive — Alembic keys on the `revision` string, not the filename.
"""

revision      = 'phase48_sched_parent_insp'
down_revision = 'phase47_schedule_end_date'
branch_labels = None
depends_on    = None

from alembic import op
import sqlalchemy as sa


_TABLE      = 'inspection_schedules'
_COLUMN     = 'parent_inspection_id'
_FK         = 'fk_inspection_schedules_parent_inspection'
_INDEX      = 'ix_inspection_schedules_parent_inspection_id'


def _table_exists(conn, table):
    return conn.execute(sa.text(
        "SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLES "
        "WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = :t"
    ), {"t": table}).scalar() > 0


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


def _constraint_exists(conn, table, name):
    return conn.execute(sa.text(
        "SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS "
        "WHERE TABLE_SCHEMA = DATABASE() "
        "AND TABLE_NAME = :t AND CONSTRAINT_NAME = :n"
    ), {"t": table, "n": name}).scalar() > 0


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


def upgrade():
    bind = op.get_bind()
    if not _table_exists(bind, _TABLE):
        return

    if not _column_exists(bind, _TABLE, _COLUMN):
        op.execute(sa.text(
            f"ALTER TABLE {_TABLE} ADD COLUMN {_COLUMN} INT NULL AFTER notes"
        ))

    # Named explicitly so the downgrade and the re-run check can find it. MySQL
    # auto-creates an index for a foreign key, but only when no usable index
    # exists; creating it first means the name is ours and predictable.
    if not _index_exists(bind, _TABLE, _INDEX):
        op.execute(sa.text(
            f"CREATE INDEX {_INDEX} ON {_TABLE} ({_COLUMN})"
        ))

    if not _constraint_exists(bind, _TABLE, _FK):
        op.execute(sa.text(
            f"ALTER TABLE {_TABLE} ADD CONSTRAINT {_FK} "
            f"FOREIGN KEY ({_COLUMN}) REFERENCES inspections(id) "
            f"ON DELETE SET NULL"
        ))


def downgrade():
    bind = op.get_bind()
    if not _table_exists(bind, _TABLE):
        return

    # FK first — MySQL refuses to drop a column or index still referenced by one.
    if _constraint_exists(bind, _TABLE, _FK):
        op.execute(sa.text(f"ALTER TABLE {_TABLE} DROP FOREIGN KEY {_FK}"))
    if _index_exists(bind, _TABLE, _INDEX):
        op.execute(sa.text(f"DROP INDEX {_INDEX} ON {_TABLE}"))
    if _column_exists(bind, _TABLE, _COLUMN):
        op.execute(sa.text(f"ALTER TABLE {_TABLE} DROP COLUMN {_COLUMN}"))