Does FastAPI Work With MongoDB?
FastAPI and MongoDB work seamlessly together, with multiple mature driver options making it straightforward to build production-ready APIs.
Quick Facts
How FastAPI Works With MongoDB
FastAPI pairs excellently with MongoDB through libraries like PyMongo (synchronous) or Motor (async). Since FastAPI's main advantage is async support, Motor is the preferred choice for non-blocking database operations. The integration is clean: you instantiate a MongoDB client at startup, pass it through FastAPI's dependency injection system, and use it in route handlers. FastAPI's Pydantic models naturally map to MongoDB documents, making validation and serialization automatic. The async nature means your API won't block on database queries, crucial for handling concurrent requests. You'll typically use Motor's AsyncIOMotorClient to connect at application startup and close at shutdown using FastAPI's lifespan context managers. The document-oriented nature of MongoDB aligns well with FastAPI's data validation philosophy, though you should still use Pydantic schemas as the source of truth for API contracts rather than relying solely on MongoDB's flexible schema.
Best Use Cases
Quick Setup
pip install fastapi motor pymongo[srv] uvicornfrom fastapi import FastAPI, Depends
from motor.motor_asyncio import AsyncIOMotorClient, AsyncIOMotorDatabase
from pydantic import BaseModel
from contextlib import asynccontextmanager
class User(BaseModel):
name: str
email: str
mongo_client: AsyncIOMotorClient = None
db: AsyncIOMotorDatabase = None
@asynccontextmanager
async def lifespan(app: FastAPI):
global mongo_client, db
mongo_client = AsyncIOMotorClient("mongodb+srv://user:pass@cluster.mongodb.net")
db = mongo_client["myapp"]
yield
mongo_client.close()
app = FastAPI(lifespan=lifespan)
async def get_db():
return db
@app.post("/users")
async def create_user(user: User, db: AsyncIOMotorDatabase = Depends(get_db)):
result = await db.users.insert_one(user.dict())
return {"id": str(result.inserted_id)}
@app.get("/users/{user_id}")
async def get_user(user_id: str, db: AsyncIOMotorDatabase = Depends(get_db)):
from bson import ObjectId
user = await db.users.find_one({"_id": ObjectId(user_id)})
return userKnown Issues & Gotchas
ObjectId serialization in JSON responses causes type errors
Fix: Convert ObjectId to string in Pydantic models using field validators or custom JSON encoders in FastAPI's JSONEncoder
Motor's async operations don't automatically handle connection pooling limits under high load
Fix: Configure maxPoolSize and minPoolSize in connection string; monitor connection usage in production
Pydantic v2 changed validation behavior, breaking some existing MongoDB field configurations
Fix: Update Pydantic models to use ConfigDict and field validators instead of deprecated Config class
MongoDB transactions require replica set deployments, not available in standalone mode
Fix: Use MongoDB Atlas or configure a local replica set for multi-document ACID transactions
Alternatives
- •Express.js with Mongoose - synchronous but simpler learning curve for Node.js teams
- •Django with MongoEngine - more batteries-included ORM approach with Python
- •Go with MongoDB Go driver - better performance for ultra-high-concurrency scenarios
Resources
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