--- name: db-mongodb description: MongoDB patterns. Schema design, document modeling, indexing, aggregation pipelines, Mongoose\/native driver patterns. --- # MongoDB Integration Patterns Production-ready patterns for MongoDB in Node.js\/TypeScript applications using native driver. ## Connection Pooling ### Lambda Connection Reuse ```typescript import { MongoClient, Db } from 'mongodb'; let cachedDb: Db | null = null; async function connectToDatabase(): Promise { if (cachedDb) { return cachedDb; } const client = await MongoClient.connect(process.env.MONGODB_URI!, { maxPoolSize: 10, minPoolSize: 2, maxIdleTimeMS: 30000, serverSelectionTimeoutMS: 5000, }); cachedDb = client.db('myapp'); return cachedDb; } \/\/ Usage in Lambda export const handler = async (event: APIGatewayEvent) => { const db = await connectToDatabase(); \/\/ Reuses connection const users = await db.collection('users').find().toArray(); return { statusCode: 200, body: JSON.stringify(users) }; }; ``` ## Type-Safe Collections ### Repository Pattern ```typescript import { Collection, ObjectId } from 'mongodb'; interface User { _id?: ObjectId; email: string; name: string; createdAt: Date; } class UserRepository { private collection: Collection; constructor(db: Db) { this.collection = db.collection('users'); } async create(user: Omit): Promise { const result = await this.collection.insertOne(user); return { ...user, _id: result.insertedId }; } async findById(id: string): Promise { return this.collection.findOne({ _id: new ObjectId(id) }); } async findByEmail(email: string): Promise { return this.collection.findOne({ email }); } async update(id: string, updates: Partial): Promise { await this.collection.updateOne( { _id: new ObjectId(id) }, { $set: updates } ); } async delete(id: string): Promise { await this.collection.deleteOne({ _id: new ObjectId(id) }); } } ``` ## Index Management ### Idempotent Index Creation ```typescript \/\/ Create indexes at startup (idempotent) async function ensureIndexes(db: Db): Promise { const users = db.collection('users'); await users.createIndex({ email: 1 }, { unique: true }); await users.createIndex({ createdAt: -1 }); await users.createIndex({ 'profile.location': 1 }, { sparse: true }); \/\/ Compound index await users.createIndex({ status: 1, createdAt: -1 }); \/\/ Text search index await users.createIndex({ name: 'text', bio: 'text' }); } ``` ## Error Handling ### Duplicate Key Errors ```typescript import { MongoError } from 'mongodb'; try { await collection.insertOne(user); } catch (error) { if (error instanceof MongoError) { \/\/ Duplicate key error if (error.code === 11000) { throw new Error('Email already exists'); } } throw error; } ``` ## Common Pitfalls ### ObjectId Conversion ```typescript \/\/ ❌ String comparison doesn't work const user = await collection.findOne({ _id: '507f1f77bcf86cd799439011' }); \/\/ ✓ Convert to ObjectId const user = await collection.findOne({ _id: new ObjectId('507f1f77bcf86cd799439011') }); ``` ### Missing Indexes ```typescript \/\/ ❌ Slow query without index const users = await collection.find({ email: 'user@example.com' }).toArray(); \/\/ ✓ Create index first await collection.createIndex({ email: 1 }, { unique: true }); const users = await collection.find({ email: 'user@example.com' }).toArray(); ```