mongrator 0.4.2

Lightweight MongoDB schema migration tool

mongrator

Lightweight MongoDB schema migration tool with synchronous and asynchronous PyMongo support.

Installation

pip install mongrator

Or with uv:

uv add mongrator

Quick start

# Create config and migrations directory
mongrator init

# Generate a new migration file
mongrator create add_users_email_index

# Check migration status
mongrator status

# Apply pending migrations
mongrator up

# Roll back the last migration
mongrator down

Configuration

mongrator init creates a mongrator.toml stub:

uri = "mongodb://localhost:27017"
database = "mydb"
migrations_dir = "migrations"
collection = "mongrator_migrations"  # optional

Alternatively, configure via environment variables:

Variable	Description	Required
MONGRATOR_URI	MongoDB connection URI	yes
MONGRATOR_DB	Database name	yes
MONGRATOR_MIGRATIONS_DIR	Path to migrations directory	no (default: migrations)
MONGRATOR_COLLECTION	Tracking collection name	no (default: mongrator_migrations)

Writing migrations

Migration files are plain Python named {timestamp}_{slug}.py (e.g. 20260408_143022_add_users_email_index.py). Each file must define an up(db) function. A down(db) function is optional but enables rollback.

Using the ops helpers (recommended)

The ops helpers record their own inverses, so down() is generated automatically:

from mongrator import ops

def up(db):
    return [
        ops.create_index("users", {"email": 1}, unique=True),
        ops.rename_field("users", "username", "handle"),
        ops.add_field("users", "verified", default_value=False),
    ]

Using plain PyMongo

For complex logic, write directly against the db argument and define down() manually:

def up(db):
    db["orders"].update_many(
        {"status": {"$exists": False}},
        {"$set": {"status": "pending"}},
    )

def down(db):
    db["orders"].update_many({}, {"$unset": {"status": ""}})

Available ops helpers

Helper	Reversible	Description
ops.create_index(collection, keys, **kwargs)	yes	Create an index
ops.drop_index(collection, index_name, keys=None, **kwargs)	keys[^1]	Drop an index by name
ops.rename_field(collection, old, new, filter=None)	yes	Rename a field across documents
ops.add_field(collection, field, default_value, filter=None)	yes	Add a field with a default value

[^1]: Only reversible when keys is provided. See writing migrations for details.

CLI reference

mongrator init                        create migrations dir and mongrator.toml
mongrator create <name>               generate a new migration file
mongrator status                      show applied/pending migrations
mongrator up [--target ID]            apply pending migrations
mongrator up --async [--target ID]    apply using async runner
mongrator down [--steps N]            roll back N migrations (default: 1)
mongrator down --async [--steps N]    roll back using async runner
mongrator validate                    verify checksums of applied migrations
mongrator --config PATH <command>     use an alternate config file

Async usage

Pass --async to up or down to use the async runner (backed by pymongo.AsyncMongoClient):

mongrator up --async

To use the runners programmatically:

# Synchronous
from pathlib import Path
import pymongo
from mongrator.config import MigratorConfig
from mongrator.runner import SyncRunner

config = MigratorConfig(uri="mongodb://localhost:27017", database="mydb", migrations_dir=Path("migrations"))
runner = SyncRunner(pymongo.MongoClient(config.uri), config)
runner.up()

# Asynchronous
from pymongo import AsyncMongoClient
from mongrator.runner import AsyncRunner

runner = AsyncRunner(AsyncMongoClient(config.uri), config)
await runner.up()

Migration tracking

Applied migrations are recorded in the mongrator_migrations collection (configurable) within the target database. Each document stores:

• _id — migration file stem
• applied_at — UTC timestamp
• checksum — SHA-256 of the migration file at time of application
• direction — "up" or "down"
• duration_ms — execution time in milliseconds

Running mongrator validate compares current file checksums against recorded values and reports any modifications.