Does Flask Work With MongoDB?
Flask and MongoDB work together seamlessly with minimal friction, making it a popular choice for building flexible, scalable web applications.
Quick Facts
How Flask Works With MongoDB
Flask and MongoDB integrate beautifully through community libraries like PyMongo (official MongoDB driver) and MongoEngine (ODM layer). PyMongo provides direct database access with full control over queries, while MongoEngine adds schema validation and ORM-like convenience. Neither requires tight coupling—you simply instantiate a MongoDB client in your Flask app and use it in route handlers or service layers. The document-oriented nature of MongoDB aligns well with Flask's lightweight philosophy: no migrations, flexible schemas that evolve with your application, and JSON-native serialization that pairs naturally with Flask's JSON responses. Most developers use PyMongo for direct control or MongoEngine for schema management, depending on whether they value flexibility or structure. The main architectural consideration is connection pooling: Flask apps should reuse a single MongoDB client instance across requests rather than creating new connections, which you typically handle via Flask's application context or a singleton pattern.
Best Use Cases
Quick Setup
pip install flask pymongofrom flask import Flask, jsonify
from pymongo import MongoClient
from bson.objectid import ObjectId
app = Flask(__name__)
client = MongoClient('mongodb://localhost:27017/')
db = client['my_app']
@app.route('/users', methods=['GET'])
def get_users():
users = list(db.users.find().limit(10))
for user in users:
user['_id'] = str(user['_id'])
return jsonify(users)
@app.route('/users/<user_id>', methods=['GET'])
def get_user(user_id):
user = db.users.find_one({'_id': ObjectId(user_id)})
if user:
user['_id'] = str(user['_id'])
return jsonify(user)
return {'error': 'Not found'}, 404
if __name__ == '__main__':
app.run(debug=True)Known Issues & Gotchas
Connection pooling exhaustion under high concurrency if connections aren't properly managed
Fix: Store MongoDB client in Flask app context using app.config or a singleton; let PyMongo handle connection pooling automatically rather than creating multiple clients
ObjectId serialization fails when converting MongoDB documents directly to JSON responses
Fix: Implement a custom JSON encoder or convert ObjectId to string: json.dumps(doc, default=str) or use a helper function in your response layer
No built-in transaction support in older MongoDB versions, leading to consistency issues in multi-document updates
Fix: Use MongoDB 4.0+ for multi-document ACID transactions, or redesign schema to keep related data in single documents
N+1 query problems when iterating over documents and fetching related data in loops
Fix: Use MongoDB aggregation pipeline or denormalization strategies; with MongoEngine, use .select_related() equivalent patterns
Alternatives
- •Django with MongoDB (django-mongodb-engine) - heavier framework with more built-in features
- •FastAPI with Motor (async MongoDB driver) - modern async alternative for high-performance APIs
- •Flask with PostgreSQL - relational structure with mature ORM ecosystem (SQLAlchemy)
Resources
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