pytest-mrt 0.4.0

Catch database migration rollback failures before they reach production

pytest-mrt

A pytest plugin that catches database migration rollback failures before they reach production.

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alembic downgrade -1 ran clean. No errors. Your monitoring went green.

But the users' phone numbers are gone. The column came back. The data didn't.

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What it does

Most tools verify that migrations run without errors.
pytest-mrt verifies that your data survives a rollback.

It seeds real rows before each migration, rolls back, and checks nothing was lost. It also statically scans migration files for 24 known dangerous patterns.

Install

pip install pytest-mrt

Setup (2 minutes)

1. Create conftest.py in your project root:

# conftest.py
import os
from pytest_mrt import MRTConfig


def pytest_configure(config):
    config._mrt_config = MRTConfig(
        alembic_ini="alembic.ini",                               # path to your alembic.ini
        db_url=os.environ.get("TEST_DATABASE_URL", "sqlite:///test.db"),  # test database
    )

2. Write a test:

# tests/test_migrations.py


def test_migrations_are_safe(mrt):
    mrt.assert_all_reversible()

3. Run:

pytest tests/test_migrations.py -s

mrt is a pytest fixture — just add it as a parameter and it works. No import needed in test files.

Static analysis (no database needed)

mrt check migrations/versions/

╭──────────┬──────────────────────────┬─────────┬────────────────────────────────────╮
│ Revision │ Pattern                  │ Sev     │ Message                            │
├──────────┼──────────────────────────┼─────────┼────────────────────────────────────┤
│ 004      │ DROP COLUMN in upgrade   │ error   │ Data permanently lost on rollback  │
│ 005      │ No-op downgrade          │ error   │ downgrade() does nothing           │
│ 006      │ INDEX without CONCURR.   │ warning │ Locks table during index build     │
╰──────────┴──────────────────────────┴─────────┴────────────────────────────────────╯
2 error(s), 1 warning(s)

What gets caught

Errors (will cause data loss or a broken rollback):

• op.drop_column() in upgrade — data is gone even if downgrade re-adds the column
• op.drop_table() in upgrade — all rows permanently lost
• TRUNCATE in migration
• def downgrade(): pass — rollback silently does nothing
• No downgrade() function
• rename_table / rename_column without reverse
• DROP VIEW without recreating in downgrade
• ALTER TYPE ... ADD VALUE (PostgreSQL ENUM) — can't roll back once rows use the new value
• Add column + migrate data + drop original in one migration

Warnings (review before deploying):

• NOT NULL without server_default
• Column type change
• Raw op.execute()
• Bulk UPDATE without a reverse UPDATE in downgrade
• ON DELETE CASCADE added
• CREATE INDEX without CONCURRENTLY (PostgreSQL)
• ADD COLUMN with DEFAULT on large tables
• CREATE UNIQUE CONSTRAINT on existing data
• DROP INDEX without recreating
• DROP CONSTRAINT without recreating
• ALTER SEQUENCE / setval
• NOT NULL via raw SQL without reverse
• NOT NULL without restoring nullable in downgrade

Databases

	Static analysis	Dynamic verification
PostgreSQL	✅	✅
SQLite	✅	✅
MySQL / MariaDB	✅	🔜 planned

Documentation

Full docs at croc100.github.io/pytest-mrt

• Getting started (step-by-step)
• All 24 patterns explained
• CLI & fixture reference

License

MIT