"""phase44 — issue internal_handler_name / internal_handler_contact,
and convergence of handler_type to NOT NULL DEFAULT 'internal'

Ports single-tenant phase41 + phase42 into the multi-tenant chain, and closes the
last divergence in the handler model.

1. Adds `internal_handler_name` and `internal_handler_contact` to `issues`,
   capturing the janitorial staff member who will handle an issue when
   handler_type == 'internal'. Distinct from `assigned_to` (the JQC User who owns
   follow-up) — the actual crew member may not be a system user. Parallels the
   existing facility_handler_* and vendor_* contact pairs.

2. Converges `handler_type` from MT's `NULL`-able column onto the single-tenant
   definition, `NOT NULL DEFAULT 'internal'`. MT's application code already
   treats NULL and 'internal' as the same thing (`handler_type or 'internal'`
   appears in the model property, the API payload and the templates), so this
   only removes a redundant second representation of "internal". Existing NULL
   rows are backfilled to 'internal' BEFORE the MODIFY, otherwise the ALTER
   fails on a table containing NULLs.

RE-RUNNABLE. Step 1 uses an INFORMATION_SCHEMA column-existence check. Step 2
cannot use one — the column already exists; what changes is its nullability — so
it checks IS_NULLABLE instead and skips when the column is already NOT NULL.
Checking only for existence there would silently make this migration a no-op on
re-run against a half-applied schema.

No batch_alter_table (MySQL).
"""

revision      = 'phase44_internal_handler'
down_revision = 'phase43_schedule_plan_fields'
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 = :t AND COLUMN_NAME = :c"
    ), {"t": table, "c": column})
    return result.scalar() > 0


def _column_is_nullable(conn, table, column):
    """True when the column exists AND is declared NULL-able.

    Returns False for a missing column so callers never attempt to MODIFY
    something that isn't there.
    """
    result = conn.execute(sa.text(
        "SELECT IS_NULLABLE FROM INFORMATION_SCHEMA.COLUMNS "
        "WHERE TABLE_SCHEMA = DATABASE() "
        "AND TABLE_NAME = :t AND COLUMN_NAME = :c"
    ), {"t": table, "c": column})
    row = result.first()
    return bool(row) and row[0] == 'YES'


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

    # ── 1. internal_handler_name ──────────────────────────────────────────
    if not _column_exists(bind, 'issues', 'internal_handler_name'):
        op.execute(sa.text(
            "ALTER TABLE issues ADD COLUMN internal_handler_name VARCHAR(100) NULL"
        ))

    # ── 2. internal_handler_contact ───────────────────────────────────────
    if not _column_exists(bind, 'issues', 'internal_handler_contact'):
        op.execute(sa.text(
            "ALTER TABLE issues ADD COLUMN internal_handler_contact VARCHAR(200) NULL"
        ))

    # ── 3. handler_type -> NOT NULL DEFAULT 'internal' ────────────────────
    # Backfill first: MODIFY ... NOT NULL fails outright if any row holds NULL.
    # This runs unconditionally (it is itself idempotent — a second run matches
    # zero rows) so the data is correct even if a previous attempt aborted
    # between the UPDATE and the ALTER.
    if _column_is_nullable(bind, 'issues', 'handler_type'):
        op.execute(sa.text(
            "UPDATE issues SET handler_type = 'internal' WHERE handler_type IS NULL"
        ))
        op.execute(sa.text(
            "ALTER TABLE issues MODIFY COLUMN handler_type "
            "ENUM('internal','facility','vendor') NOT NULL DEFAULT 'internal'"
        ))


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

    # Revert handler_type to nullable with no default. Stored values are left
    # as-is: rows that were NULL before the upgrade are now 'internal', which is
    # semantically identical under the application's `handler_type or 'internal'`
    # reads, so there is nothing to undo in the data.
    if not _column_is_nullable(bind, 'issues', 'handler_type'):
        op.execute(sa.text(
            "ALTER TABLE issues MODIFY COLUMN handler_type "
            "ENUM('internal','facility','vendor') NULL"
        ))

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

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