"""Migration hygiene (TEST-3), DB-lane.

Two things the prod boot never checks:
- **model <-> schema drift:** `entrypoint.sh` only ever runs `alembic upgrade head`; nothing verifies the
  migrated schema still matches `Base.metadata`. A model column added without a migration (or a migration
  that drifted from the model) ships silently.
- **the downgrade path:** every migration defines `downgrade()`, but it never runs, so a broken downgrade
  is only discovered during an emergency rollback.

Both use the `db` fixture, which has already brought the schema to head via `_schema`.

Scope note: we assert on STRUCTURAL drift only — tables and columns present in the model but not the
schema, or vice versa. `compare_metadata` also reports index/constraint/sequence/type differences, but
those are dominated by migration-only objects (partial/expression/GIN indexes, CHECK constraints, SERIAL
sequences) that can't be expressed on `Base.metadata`, so comparing them wholesale is noise. Missing/extra
columns and tables — the "someone changed a model without a migration" case — are the signal worth gating.
"""
from alembic import command
from alembic.autogenerate import compare_metadata
from alembic.config import Config
from alembic.migration import MigrationContext

from app.db import Base, engine

# The diff ops that mean a real table/column is present on one side and not the other.
_DRIFT_OPS = {"add_table", "remove_table", "add_column", "remove_column"}


def _structural_drift():
    """Table/column presence diffs between the ORM models and the migrated schema (`compare_type=True`
    mirrors alembic/env.py). compare_metadata nests per-column ops in a sub-list per table and returns
    table-level ops as bare tuples; flatten both and keep only the presence ops."""
    with engine.connect() as conn:
        ctx = MigrationContext.configure(conn, opts={"compare_type": True})
        diffs = compare_metadata(ctx, Base.metadata)
    drift = []
    for d in diffs:
        for entry in (d if isinstance(d, list) else [d]):
            if isinstance(entry, tuple) and entry and entry[0] in _DRIFT_OPS:
                drift.append(entry)
    return drift


def test_models_match_the_migrated_schema(db):
    # Non-empty == the ORM models and the alembic-built schema disagree on a table/column — almost
    # always a model change that forgot its migration.
    drift = _structural_drift()
    assert drift == [], "model/schema drift — add or fix a migration:\n" + "\n".join(
        str(d) for d in drift
    )


def test_head_migration_downgrades_and_reupgrades(db):
    # Exercise the newest migration's downgrade + re-upgrade so a broken downgrade is caught the sprint
    # it lands, not during a rollback. Leaves the schema back at head for the rest of the session.
    cfg = Config("alembic.ini")
    command.downgrade(cfg, "-1")
    command.upgrade(cfg, "head")
    assert _structural_drift() == []