3. MongoDB

A comprehensive MongoDB guide covering core concepts, advanced features, and how it handles things like foreign keys, relationships, and beyond — all in one place.

📘 MongoDB Comprehensive Guide

1. 🔹 What is MongoDB?

MongoDB is a NoSQL database that stores data in JSON-like documents (BSON).
It’s schema-less by default — meaning documents in a collection can have different structures.
Used for scalable, high-performance apps — especially in Node.js/Express ecosystems.

2. 🔹 Basic Building Blocks

Concept	MongoDB Equivalent
Database	Database
Table	Collection
Row	Document
Column	Field
Primary Key	`_id` field (auto-gen)

3. 🔹 Data Types

MongoDB supports:

String, Number, Boolean
Array, Object
Date
ObjectId (unique identifier)
Null

4. 🔹 CRUD Operations

🔸 Create

db.users.insertOne({ name: "Alice", age: 30 })

🔸 Read

db.users.find({ name: "Alice" })

🔸 Update

db.users.updateOne({ name: "Alice" }, { $set: { age: 31 } })

🔸 Delete

db.users.deleteOne({ name: "Alice" })

5. 🔹 Schema Design (Mongoose)

You can enforce structure using Mongoose in Node.js:

const userSchema = new mongoose.Schema({
  name: String,
  email: { type: String, unique: true },
  age: Number
});

6. 🔹 Relationships in MongoDB

MongoDB is non-relational, but you can model relationships in two ways:

A. Embedding (Denormalization)

{
  name: "Alice",
  orders: [
    { product: "Book", quantity: 2 },
    { product: "Pen", quantity: 5 }
  ]
}

✅ Great for fast reads, fewer joins.

B. Referencing (Normalization)

// users
{ _id: ObjectId("u1"), name: "Alice" }

// orders
{ userId: ObjectId("u1"), product: "Book" }

You can manually “join” with populate() in Mongoose:

Order.find().populate('userId');

✨ How it works:

.populate('userId') works only if you’ve told Mongoose how userId is related in your schema.

So the key is setting up the reference properly.

✅ Example: Setting Up the Relationship

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  email: String
});

module.exports = mongoose.model('User', userSchema);

const mongoose = require('mongoose');

const orderSchema = new mongoose.Schema({
  product: String,
  userId: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User'  // <--- THIS tells Mongoose to link with the User model
  }
});

module.exports = mongoose.model('Order', orderSchema);

Now when you run this:

Order.find().populate('userId')

Mongoose:

Sees userId is a reference (ref: 'User')
Looks up the User model
Replaces userId with the actual User document

🧪 Output Before `.populate()`

{
  _id: "order1",
  product: "Book",
  userId: "662ac3f9d25a10a9f1b821ad"
}

✅ Output After `.populate()`

{
  _id: "order1",
  product: "Book",
  userId: {
    _id: "662ac3f9d25a10a9f1b821ad",
    name: "Alice",
    email: "alice@example.com"
  }
}

⚠️ Common Mistakes

❌ Forgetting ref: 'User' in the schema
❌ Using string instead of mongoose.Schema.Types.ObjectId
❌ The referenced document (User) doesn’t exist
❌ Typo in the model name (ref: 'user' instead of 'User')

7. 🔹 Does MongoDB Support Foreign Keys?

Not natively like SQL.

MongoDB:

Doesn’t enforce foreign keys.
You can manually manage relations and use tools like Mongoose to simulate joins.

In Mongoose:

const orderSchema = new mongoose.Schema({
  userId: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
  product: String
});

8. 🔹 Indexes

Indexes improve query performance.

db.users.createIndex({ email: 1 }, { unique: true });