Mongodb Embedding Vs Referencing Documents Complete Guide

Example 1: Embedding Documents

Scenario: Let's consider a blog application where each post has several comments. Comments are usually not as big as posts and do not need to be accessed independently. In this case, embedding the comments within the post documents is a good strategy.

Step 1: Define the Schema

In an embedded document schema, the blog post has a comments array where each comment is stored directly inside the post document.

{
    "_id": ObjectId("..."),
    "title": "MongoDB Performance",
    "content": "Learn about MongoDB indexing, query optimizations, and more...",
    "comments": [
        {
            "author": "John Doe",
            "text": "Great post!",
            "date": ISODate("2023-10-01T12:00:00Z")
        },
        {
            "author": "Jane Smith",
            "text": "Very helpful thanks for sharing.",
            "date": ISODate("2023-10-01T12:30:00Z")
        }
    ]
}

Step 2: Insert Data

Insert a blog post with some comments into your MongoDB database.

Using MongoDB shell:

db.posts.insertOne({
    title: "MongoDB Performance",
    content: "Learn about MongoDB indexing, query optimizations, and more...",
    comments: [
        { author: "John Doe", text: "Great post!", date: new Date() },
        { author: "Jane Smith", text: "Very helpful thanks for sharing.", date: new Date() }
    ]
});

Step 3: Query Data

To fetch all posts with their respective comments, a simple find operation on the posts collection will suffice.

db.posts.find().pretty();

Output:

{
    "_id": ObjectId(...),
    "title": "MongoDB Performance",
    "content": "Learn about MongoDB indexing, query optimizations, and more...",
    "comments": [
        { "author": "John Doe", "text": "Great post!", "date": ISODate(...) },
        { "author": "Jane Smith", "text": "Very helpful thanks for sharing.", "date": ISODate(...) }
    ]
}

Example 2: Referencing Documents

Scenario: Consider another example of a blog application where each user can have multiple blog posts, but also might comment on multiple posts. Both posts and comments need to be accessed independently.

Step 1: Define the Schema

In a referenced document schema, we will have separate collections for users, posts, and comments:

• Users Collection: Stores details about each user.
• Posts Collection: Contains posts made by users, references user documents.
• Comments Collection: Holds the comments on various posts, references both user and post documents.

Schema Definition

// Users Collection
{
    "_id": ObjectId("..."),
    "username": "john_doe",
    "email": "john@example.com"
}

// Posts Collection
{
    "_id": ObjectId("..."),
    "title": "MongoDB Performance",
    "content": "Learn about MongoDB indexing, query optimizations, and more...",
    "author_id": ObjectId("..."), // Reference to Users collection
    "post_date": ISODate("2023-10-01T10:00:00Z")
}

// Comments Collection
{
    "_id": ObjectId("..."),
    "post_id": ObjectId("..."), // Reference to Posts collection
    "author_id": ObjectId("..."), // Reference to Users collection
    "text": "Great post!",
    "comment_date": ISODate("2023-10-01T12:00:00Z")
}

Step 2: Insert Data

We will first insert a user, then a post associated with that user, and finally, one or more comments linked to the post as well as the user.

Using MongoDB shell:

// Insert user document
var johnId = db.users.insertOne({ username: "john_doe", email: "john@example.com" }).insertedId;

// Insert post document
var postId = db.posts.insertOne({
    title: "MongoDB Performance",
    content: "Learn about MongoDB indexing, query optimizations, and more...",
    author_id: johnId,
    post_date: new Date()
}).insertedId;

// Insert comment document
db.comments.insertOne({
    post_id: postId,
    author_id: johnId,
    text: "Great post!",
    comment_date: new Date()
});

db.comments.insertOne({
    post_id: postId,
    author_id: ObjectId("...", username: "jane_smith"), // assuming jane already exists
    text: "Very helpful thanks for sharing.",
    comment_date: new Date()
});

Step 3: Query Data

Fetching all posts along with their respective authors and comments involves multiple steps such as querying each collection and joining data based on references.

// Fetch a single post with its author and comments
var post = db.posts.findOne({ _id: ObjectId("...") });

var author = db.users.findOne({ _id: post.author_id });
    
var comments = db.comments.find({ post_id: post._id }).toArray();

console.log(post);
console.log(author);
console.log(comments);

Output: