Embedded Documents

Introduction

Embedding is the act of storing related data inside its parent document instead of a separate collection. It's the most common modeling pattern in MongoDB and the source of most performance wins over SQL.

Understanding the topic

Embedding rule of thumb:

• Data is owned by the parent (address, settings).
• Always read together.
• Bounded size (≤ ~MB, ≤ ~hundreds of elements).
• Doesn't need to be queried alone across parents.

Syntax reference

// Embed
{ _id: 1, user: "Ada", profile: { bio: "...", avatar: "..." } }

// Versus reference
{ _id: 1, user: "Ada", profileId: ObjectId("...") }

Real-world use

Order line items are almost always embedded — they're created with the order, read with the order, and immutable after checkout.

Best practices

• Embed when ownership + locality + bounded size align.
• Reference otherwise.