Nested Documents

Introduction

MongoDB lets you nest documents inside documents to any reasonable depth. This is one of its superpowers: an order with line items, a user with an address, a post with comments — all fit in a single document, queryable in one round-trip.

Understanding the topic

When to nest (embed):

• The nested data is owned by the parent (a profile's address).
• You always read them together.
• They don't grow unbounded (cap at ~hundreds of sub-docs).
• You don't need to query them independently across many parents.

Syntax reference

db.users.insertOne({
  name: "Ada",
  address: { street: "1 King's Way", city: "London", zip: "EC1A" }
});

// Query nested with dot notation
db.users.find({ "address.city": "London" });

Real-world use

User profile + address + preferences = one document. UI loads in one query, no joins.

Best practices

• Embed 1:1 and 1:few; reference 1:many.
• Index nested fields you query (address.city).

Common mistakes

• Embedding unbounded arrays (e.g. all of a user's posts) — eventually hits 16 MB.