Arrays in MongoDB

Introduction

Arrays are first-class citizens in MongoDB. You can store them, query them, project specific elements, and atomically push/pull/update items. Most NoSQL databases treat arrays as opaque blobs — MongoDB lets you reach inside them with surgical precision.

Understanding the topic

Powerful array operators:

• $push / $pull — atomic add/remove.
• $addToSet — push only if absent (set semantics).
• $elemMatch — match an array element by multiple criteria.
• $size — match by array length.
• arrayFilters — update specific matching elements.

Syntax reference

// Tag a post
db.posts.updateOne({ _id: 1 }, { $addToSet: { tags: "mongodb" } });

// Find users with at least one admin role
db.users.find({ roles: { $elemMatch: { $eq: "admin" } } });

// Increment one variant's stock
db.products.updateOne(
  { _id: 1, "variants.color": "blue" },
  { $inc: { "variants.$.stock": 5 } }
);

Real-world use

Tags on a blog post, line items on an order, attendees on an event — all idiomatic array use cases.

Best practices

• Cap arrays at ~hundreds of elements.
• Use multikey indexes for queried array fields.

Common mistakes

• Storing millions of items in one array — use a separate collection.