🔗 Creating Relationships in MongoDB Using Mongoose

MongoDB is a NoSQL database and does not have traditional foreign key relationships like relational databases. However, with Mongoose (an ODM for MongoDB in Node.js), you can create and manage relationships between documents using references and population.

🧰 Types of Relationships in Mongoose

There are two main types of relationships:

Referenced (Normalization)

Documents reference each other by _id.

Good for large datasets and when documents are frequently updated.

Embedded (Denormalization)

One document contains another as a subdocument.

Good for nested, read-heavy data.

📘 Example: Users and Posts

Let’s walk through creating a one-to-many relationship:

One User → Many Posts

📁 1. Define the Mongoose Schemas

User Schema (referenced style)

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const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const UserSchema = new Schema({

  name: String,

  email: String,

});

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

Post Schema (referencing user by ID)

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const PostSchema = new Schema({

  title: String,

  content: String,

  author: { type: Schema.Types.ObjectId, ref: 'User' },

});

module.exports = mongoose.model('Post', PostSchema);

🧪 2. Creating and Saving Documents

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const User = require('./models/User');

const Post = require('./models/Post');

async function createPost() {

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

  const post = await Post.create({

    title: 'My First Post',

    content: 'This is a post written by Alice.',

    author: user._id,

  });

  console.log('Post created:', post);

}

🔍 3. Populating the Author Field

To retrieve the post with the user data instead of just the user._id:

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const post = await Post.findOne({ title: 'My First Post' }).populate('author');

console.log(post);

Output includes full author info:

json

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{

  "title": "My First Post",

  "author": {

    "_id": "60...",

    "name": "Alice",

    "email": "alice@example.com"

  }

}

🧩 Embedded Documents (Alternative)

In some cases, especially for comments or settings, you may want to embed the data.

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const CommentSchema = new Schema({

  body: String,

  postedAt: Date

});

const PostSchema = new Schema({

  title: String,

  content: String,

  comments: [CommentSchema]

});

With this approach, comments are stored directly inside the Post document.

✅ When to Use Which

Type Use When...

Referenced Data is large or changes frequently

Embedded Data is small, tightly coupled, and read together

📌 Summary

Use ObjectId references and .populate() for normalized relationships.

Use embedded documents for nested, always-accessed-together data.

Mongoose makes it easy to model relational patterns in a non-relational database.

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