pytest-mrt

Migration Rollback Tester — A pytest plugin that catches database migration disasters before they reach production.

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The problem

You run alembic downgrade -1. It succeeds. No errors.

But your users' data is gone.

The column structure came back. The rows didn't.

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This is one of the most common sources of production incidents with database migrations. alembic downgrade can succeed while silently destroying everything it was supposed to restore — because most tools only check that the migration runs, not that your data survives.

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How pytest-mrt helps

pytest-mrt does two things:

1. Static analysis — scans your migration files for 24 known dangerous patterns before you touch a database:

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             │
└──────────┴─────────────────────────┴───────┴──────────────────────────────────────┘
2 error(s)

2. Dynamic verification — seeds real data, runs your migration, rolls it back, and checks nothing was lost:

# tests/test_migrations.py

def test_migrations_are_safe(mrt):
    mrt.assert_all_reversible()

  ✓  001  reversible
  ✓  002  reversible
  ✗  003  data loss detected
     └─ Table 'users': 3/3 rows lost after rollback

  FAILED

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Install

pip install pytest-mrt

Because it's a pytest plugin, the mrt fixture is automatically available in all your tests once installed. No imports needed in test files.

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5-minute setup

1. Install:

pip install pytest-mrt

2. Create conftest.py:

from pytest_mrt import MRTConfig

def pytest_configure(config):
    config._mrt_config = MRTConfig(
        alembic_ini="alembic.ini",
        db_url="postgresql://localhost/myapp_test",
    )

3. Write a test:

def test_migrations_are_safe(mrt):
    mrt.assert_all_reversible()

4. Run:

pytest tests/test_migrations.py -s

→ Full setup guide

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Supported databases

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

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Why not just read the migration file manually?

You could — but you'd have to know all 24 patterns, remember to check every PR, and still wouldn't catch the cases that only appear when real data is present. pytest-mrt does it automatically on every test run.

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Get started → See all patterns →