import { MongoClient, Db, Collection } from 'mongodb';

// Connection URI
const uri = process.env.MONGODB_URI || 'mongodb://localhost:27017';
const client = new MongoClient(uri);

// Singleton pattern
let db: Db;

export async function connectToDatabase(): Promise<Db> {
  if (db) return db;

  try {
    await client.connect();
    db = client.db('myapp');
    console.log('Connected to MongoDB');
    return db;
  } catch (error) {
    console.error('MongoDB connection error:', error);
    throw error;
  }
}

// Get typed collection
export function getCollection<T>(name: string): Collection<T> {
  return db.collection<T>(name);
}

// Graceful shutdown
process.on('SIGINT', async () => {
  await client.close();
  process.exit(0);
});

CRUD Operations

import { ObjectId, Filter, UpdateFilter } from 'mongodb';

interface User {
  _id?: ObjectId;
  name: string;
  email: string;
  age: number;
  createdAt: Date;
  tags?: string[];
}

const users = getCollection<User>('users');

// Create
async function createUser(data: Omit<User, '_id'>): Promise<User> {
  const result = await users.insertOne({
    ...data,
    createdAt: new Date(),
  });

  return { _id: result.insertedId, ...data, createdAt: new Date() };
}

// Create many
async function createUsers(data: Omit<User, '_id'>[]): Promise<ObjectId[]> {
  const result = await users.insertMany(
    data.map(d => ({ ...d, createdAt: new Date() }))
  );
  return Object.values(result.insertedIds);
}

// Read one
async function getUserById(id: string): Promise<User | null> {
  return users.findOne({ _id: new ObjectId(id) });
}

// Read with query
async function findUsers(filter: Filter<User>): Promise<User[]> {
  return users.find(filter).toArray();
}

// Read with options
async function searchUsers(
  query: string,
  page: number = 1,
  limit: number = 10
): Promise<{ users: User[]; total: number }> {
  const filter: Filter<User> = {
    $or: [
      { name: { $regex: query, $options: 'i' } },
      { email: { $regex: query, $options: 'i' } },
    ],
  };

  const [users, total] = await Promise.all([
    users
      .find(filter)
      .sort({ createdAt: -1 })
      .skip((page - 1) * limit)
      .limit(limit)
      .toArray(),
    users.countDocuments(filter),
  ]);

  return { users, total };
}

// Update one
async function updateUser(
  id: string,
  update: Partial<User>
): Promise<User | null> {
  const result = await users.findOneAndUpdate(
    { _id: new ObjectId(id) },
    { $set: update },
    { returnDocument: 'after' }
  );
  return result;
}

// Update many
async function activateUsers(ids: string[]): Promise<number> {
  const result = await users.updateMany(
    { _id: { $in: ids.map(id => new ObjectId(id)) } },
    { $set: { active: true } }
  );
  return result.modifiedCount;
}

// Delete
async function deleteUser(id: string): Promise<boolean> {
  const result = await users.deleteOne({ _id: new ObjectId(id) });
  return result.deletedCount === 1;
}

// Upsert
async function upsertUser(
  email: string,
  data: Partial<User>
): Promise<User | null> {
  const result = await users.findOneAndUpdate(
    { email },
    { $set: data, $setOnInsert: { createdAt: new Date() } },
    { upsert: true, returnDocument: 'after' }
  );
  return result;
}

Query Operators

// Comparison
const adults = await users.find({ age: { $gte: 18 } }).toArray();
const teens = await users.find({ age: { $gte: 13, $lt: 20 } }).toArray();
const specificAges = await users.find({ age: { $in: [18, 21, 25] } }).toArray();

// Logical
const result = await users.find({
  $and: [
    { age: { $gte: 18 } },
    { email: { $exists: true } }
  ]
}).toArray();

const orResult = await users.find({
  $or: [
    { name: 'Alice' },
    { email: /admin/i }
  ]
}).toArray();

// Element
const withTags = await users.find({ tags: { $exists: true } }).toArray();
const typeCheck = await users.find({ age: { $type: 'number' } }).toArray();

// Array
const hasTag = await users.find({ tags: 'developer' }).toArray();
const allTags = await users.find({ tags: { $all: ['developer', 'admin'] } }).toArray();
const arraySize = await users.find({ tags: { $size: 3 } }).toArray();
const elemMatch = await users.find({
  addresses: { $elemMatch: { city: 'NYC', zip: { $exists: true } } }
}).toArray();

// Text search (requires text index)
const textResults = await users.find({ $text: { $search: 'developer' } }).toArray();

// Regex
const pattern = await users.find({ name: { $regex: /^A/i } }).toArray();

Update Operators

// $set - Set field value
await users.updateOne(
  { _id: new ObjectId(id) },
  { $set: { name: 'New Name', 'address.city': 'NYC' } }
);

// $unset - Remove field
await users.updateOne(
  { _id: new ObjectId(id) },
  { $unset: { temporaryField: '' } }
);

// $inc - Increment
await users.updateOne(
  { _id: new ObjectId(id) },
  { $inc: { loginCount: 1, 'stats.views': 5 } }
);

// $push - Add to array
await users.updateOne(
  { _id: new ObjectId(id) },
  { $push: { tags: 'newTag' } }
);

// $push with modifiers
await users.updateOne(
  { _id: new ObjectId(id) },
  {
    $push: {
      notifications: {
        $each: [{ message: 'Hello' }],
        $position: 0,
        $slice: 10  // Keep only 10 newest
      }
    }
  }
);

// $pull - Remove from array
await users.updateOne(
  { _id: new ObjectId(id) },
  { $pull: { tags: 'oldTag' } }
);

// $addToSet - Add unique to array
await users.updateOne(
  { _id: new ObjectId(id) },
  { $addToSet: { tags: 'uniqueTag' } }
);

// $rename - Rename field
await users.updateOne(
  { _id: new ObjectId(id) },
  { $rename: { oldName: 'newName' } }
);

Aggregation Pipeline

// Basic aggregation
const stats = await users.aggregate([
  { $match: { active: true } },
  { $group: {
    _id: '$country',
    count: { $sum: 1 },
    avgAge: { $avg: '$age' },
    names: { $push: '$name' }
  }},
  { $sort: { count: -1 } },
  { $limit: 10 }
]).toArray();

// Lookup (join)
const ordersWithUsers = await db.collection('orders').aggregate([
  { $lookup: {
    from: 'users',
    localField: 'userId',
    foreignField: '_id',
    as: 'user'
  }},
  { $unwind: '$user' },
  { $project: {
    orderId: 1,
    total: 1,
    'user.name': 1,
    'user.email': 1
  }}
]).toArray();

// Faceted search
const faceted = await products.aggregate([
  { $match: { category: 'electronics' } },
  { $facet: {
    byBrand: [
      { $group: { _id: '$brand', count: { $sum: 1 } } },
      { $sort: { count: -1 } }
    ],
    byPriceRange: [
      { $bucket: {
        groupBy: '$price',
        boundaries: [0, 100, 500, 1000, Infinity],
        default: 'Other',
        output: { count: { $sum: 1 } }
      }}
    ],
    results: [
      { $skip: 0 },
      { $limit: 20 }
    ]
  }}
]).toArray();

Indexes

// Create indexes
await users.createIndex({ email: 1 }, { unique: true });
await users.createIndex({ name: 'text', bio: 'text' }); // Text index
await users.createIndex({ location: '2dsphere' }); // Geospatial
await users.createIndex({ createdAt: 1 }, { expireAfterSeconds: 86400 }); // TTL
await users.createIndex({ category: 1, price: -1 }); // Compound

// List indexes
const indexes = await users.indexes();

// Drop index
await users.dropIndex('email_1');

Transactions

const session = client.startSession();

try {
  await session.withTransaction(async () => {
    // All operations in this block are part of the transaction
    await users.updateOne(
      { _id: fromUserId },
      { $inc: { balance: -amount } },
      { session }
    );

    await users.updateOne(
      { _id: toUserId },
      { $inc: { balance: amount } },
      { session }
    );

    await transactions.insertOne({
      from: fromUserId,
      to: toUserId,
      amount,
      createdAt: new Date()
    }, { session });
  });
} finally {
  await session.endSession();
}

Mongoose ODM

Schema Definition

import mongoose, { Schema, Document, Model } from 'mongoose';

// Interface
interface IUser extends Document {
  name: string;
  email: string;
  password: string;
  age?: number;
  role: 'user' | 'admin';
  createdAt: Date;
  updatedAt: Date;

  // Methods
  comparePassword(candidate: string): Promise<boolean>;
}

// Schema
const userSchema = new Schema<IUser>(
  {
    name: {
      type: String,
      required: [true, 'Name is required'],
      trim: true,
      minlength: 2,
      maxlength: 50,
    },
    email: {
      type: String,
      required: true,
      unique: true,
      lowercase: true,
      match: [/^\S+@\S+\.\S+$/, 'Invalid email'],
    },
    password: {
      type: String,
      required: true,
      minlength: 8,
      select: false, // Don't return by default
    },
    age: {
      type: Number,
      min: 0,
      max: 150,
    },
    role: {
      type: String,
      enum: ['user', 'admin'],
      default: 'user',
    },
  },
  {
    timestamps: true,
    toJSON: {
      transform: (doc, ret) => {
        ret.id = ret._id;
        delete ret._id;
        delete ret.__v;
        delete ret.password;
        return ret;
      },
    },
  }
);

// Index
userSchema.index({ email: 1 });
userSchema.index({ name: 'text' });

// Pre-save hook
userSchema.pre('save', async function (next) {
  if (!this.isModified('password')) return next();
  this.password = await bcrypt.hash(this.password, 12);
  next();
});

// Instance method
userSchema.methods.comparePassword = async function (
  candidate: string
): Promise<boolean> {
  return bcrypt.compare(candidate, this.password);
};

// Static method
userSchema.statics.findByEmail = function (email: string) {
  return this.findOne({ email });
};

// Virtual
userSchema.virtual('posts', {
  ref: 'Post',
  localField: '_id',
  foreignField: 'author',
});

// Model
const User: Model<IUser> = mongoose.model('User', userSchema);
export default User;

Connection

import mongoose from 'mongoose';

const MONGODB_URI = process.env.MONGODB_URI!;

// Connection options
const options = {
  maxPoolSize: 10,
  serverSelectionTimeoutMS: 5000,
  socketTimeoutMS: 45000,
};

// Connect
async function connect() {
  try {
    await mongoose.connect(MONGODB_URI, options);
    console.log('MongoDB connected');
  } catch (error) {
    console.error('MongoDB connection error:', error);
    process.exit(1);
  }
}

// Event handlers
mongoose.connection.on('disconnected', () => {
  console.log('MongoDB disconnected');
});

mongoose.connection.on('error', (err) => {
  console.error('MongoDB error:', err);
});

// Graceful shutdown
process.on('SIGINT', async () => {
  await mongoose.connection.close();
  process.exit(0);
});

export default connect;

CRUD with Mongoose

import User from './models/User';

// Create
const user = await User.create({
  name: 'Alice',
  email: 'alice@example.com',
  password: 'securepassword',
});

// Or
const user = new User({ name: 'Alice', email: 'alice@example.com' });
await user.save();

// Read
const user = await User.findById(id);
const user = await User.findOne({ email });
const users = await User.find({ role: 'admin' });

// With query builder
const users = await User.find()
  .where('age').gte(18)
  .where('role').equals('user')
  .select('name email')
  .sort('-createdAt')
  .skip(0)
  .limit(10)
  .lean(); // Returns plain objects (faster)

// Populate references
const posts = await Post.find()
  .populate('author', 'name email')
  .populate({
    path: 'comments',
    populate: { path: 'user', select: 'name' }
  });

// Update
const user = await User.findByIdAndUpdate(
  id,
  { name: 'New Name' },
  { new: true, runValidators: true }
);

// Or
await User.updateOne({ _id: id }, { $set: { name: 'New Name' } });
await User.updateMany({ role: 'guest' }, { $set: { role: 'user' } });

// Delete
await User.findByIdAndDelete(id);
await User.deleteOne({ _id: id });
await User.deleteMany({ inactive: true });

Validation

const productSchema = new Schema({
  name: {
    type: String,
    required: [true, 'Product name is required'],
    validate: {
      validator: (v: string) => v.length >= 3,
      message: 'Name must be at least 3 characters',
    },
  },
  price: {
    type: Number,
    required: true,
    min: [0, 'Price cannot be negative'],
    validate: {
      validator: Number.isFinite,
      message: '{VALUE} is not a valid price',
    },
  },
  sku: {
    type: String,
    validate: {
      validator: async function (v: string) {
        const count = await mongoose.models.Product.countDocuments({ sku: v });
        return count === 0;
      },
      message: 'SKU must be unique',
    },
  },
});

// Custom error handling
try {
  await product.save();
} catch (error) {
  if (error instanceof mongoose.Error.ValidationError) {
    const messages = Object.values(error.errors).map((e) => e.message);
    console.log('Validation errors:', messages);
  }
}

Reference Files

aggregation.md - Advanced aggregation pipelines
performance.md - Optimization and indexing strategies

mongodb

MongoDB

Quick Start

Native Driver

Connection