NoSQL: Document Stores

Document databases store JSON-like documents instead of rows. Each document can have a different shape, making them ideal for data with evolving schemas — user profiles, content, product catalogs, and mobile app backends where the fields vary by type or version. MongoDB and Firestore are the most widely used document stores. The key difference from relational databases: there are no JOINs. You either embed related data inside the document (fast reads, larger documents) or reference it by ID and fetch separately (normalized, slower). The right choice depends on how you access the data — embed things you always read together.

const user = await db.users.findOne({ id });
const posts = await db.posts.find({ userId: id });

// User document with embedded recent posts
{
  "_id": "user_123",
  "name": "Alice",
  "email": "alice@example.com",
  "recentPosts": [
    { "title": "My First Post", "publishedAt": "2026-06-01" },
    { "title": "On Databases",  "publishedAt": "2026-06-15" }
  ]
}