FerretDB — Open Source MongoDB Alternative on PostgreSQL

What it is

FerretDB is a truly open-source MongoDB-compatible database. It accepts MongoDB wire protocol connections and translates those queries into SQL, executing them against PostgreSQL or SQLite as the storage backend.

FerretDB targets teams that depend on MongoDB drivers and tools but want to avoid the SSPL license or vendor lock-in. If you already run PostgreSQL, FerretDB lets you keep your MongoDB application code while storing data in a fully relational engine.

How it saves time or tokens

Migrating from MongoDB to PostgreSQL usually means rewriting every query, changing drivers, and rethinking schema design. FerretDB eliminates that rewrite by acting as a transparent proxy. Your application connects to FerretDB using a standard MongoDB connection string, and FerretDB handles the SQL translation internally.

For AI agents that generate database queries, FerretDB means they can produce familiar MongoDB syntax while the data sits in PostgreSQL, reducing the complexity of prompt engineering for database operations.

How to use

1. Start FerretDB with Docker Compose:

# docker-compose.yml
services:
  ferretdb:
    image: ghcr.io/ferretdb/ferretdb
    ports:
      - '27017:27017'
    environment:
      FERRETDB_POSTGRESQL_URL: postgres://user:pass@postgres:5432/ferretdb
  postgres:
    image: postgres:16
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: ferretdb

1. Connect using any MongoDB driver or shell:

mongosh 'mongodb://localhost:27017/'

1. Use standard MongoDB operations:

db.products.insertOne({ name: 'Widget', price: 9.99 })
db.products.find({ price: { $lt: 20 } })

FerretDB translates these into PostgreSQL queries under the hood.

Example

// Standard MongoDB aggregation pipeline
db.orders.aggregate([
  { $match: { status: 'completed' } },
  { $group: { _id: '$region', total: { $sum: '$amount' } } },
  { $sort: { total: -1 } }
])

This runs as a SQL query against PostgreSQL, returning results in MongoDB document format.

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Common pitfalls

• Expecting full MongoDB feature parity. FerretDB covers core CRUD and aggregation but some advanced features like change streams or sharding are not yet supported. Check the compatibility matrix before migrating.
• Using FerretDB for new greenfield projects. If you do not have existing MongoDB code, using PostgreSQL directly with a proper ORM is simpler and avoids the translation overhead.
• Ignoring PostgreSQL tuning. FerretDB performance depends on PostgreSQL configuration. Default settings may not handle the document-style workload patterns well.
• Failing to review community discussions and changelogs before upgrading. Breaking changes in major versions can disrupt existing workflows. Pin versions in production and test upgrades in staging first.