Document Structure

Introduction

Every MongoDB document is a tree of key-value pairs. Keys are strings, values are any BSON type — including arrays and other documents. Mastering document structure is the foundation of every data model decision you'll ever make.

Understanding the topic

The building blocks:

• Primitive fields — strings, numbers, booleans, dates.
• Arrays — ordered lists, queryable with operators like $elemMatch.
• Embedded documents — nested objects, dot-path addressable.
• Mixed types — the same field can hold different types across documents (avoid in practice).
• Reserved _id — primary key, immutable once set.

Syntax reference

{
  _id: ObjectId("..."),
  name: "Ada",
  age: 36,
  active: true,
  tags: ["admin", "engineer"],
  address: { city: "London", postcode: "EC1A 1AA" },
  meta: { lastLogin: ISODate("2026-06-01") }
}

Real-world use

A well-structured document maps 1:1 to your API response — no joins, no transformations in the application layer.

Best practices

• Keep field names short — they're stored on every document.
• Avoid mixed types per field — kills query optimization.

Common mistakes

• Using deeply nested structures (>4 levels) — hard to query and update.