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MongoDB's Three-Level Hierarchy

What Is a Namespace?

// Namespace: database.collection
// 'shop' database, 'products' collection:
use shop
db.products.find({})
// MongoDB internally refers to this as 'shop.products'

// You can also reference it explicitly:
db.getSiblingDB('shop').getCollection('products').find({})

Switching and Creating Databases

// Switch to (or create) a database
use myNewApp
// 'myNewApp' doesn't exist yet - no error, but not visible in 'show dbs'

// Insert a document to materialize the database
db.users.insertOne({ name: 'Alice' });

// Now 'myNewApp' appears:
show dbs
// admin     0.000GB
// local     0.000GB
// myNewApp  0.000GB

System Databases: admin, local, config

// View all databases on the server
show dbs
// admin   0.000GB
// config  0.000GB
// local   0.004GB
// shop    0.012GB
// myapp   0.008GB

// List databases programmatically
db.adminCommand({ listDatabases: 1 });

Creating Collections Explicitly

// Create a collection with a JSON Schema validator
db.createCollection('products', {
  validator: {
    $jsonSchema: {
      bsonType: 'object',
      required: ['name', 'price'],
      properties: {
        name:  { bsonType: 'string' },
        price: { bsonType: 'number', minimum: 0 }
      }
    }
  }
});

Listing and Inspecting Collections

use shop

show collections
// orders
// products
// users

db.getCollectionInfos();
// Returns array of {name, type, options, idIndex} per collection

db.orders.stats().count;
// 15234

db.orders.totalIndexSize();
// 2359296 (bytes)

Renaming and Dropping Collections

// Rename a collection
db.oldUsers.renameCollection('users');
// { ok: 1 }

// Drop a collection - IRREVERSIBLE
db.tempData.drop();
// true

// Drop the entire database - EXTREMELY DANGEROUS
// db.dropDatabase();
// Use only in development or with full backup

Multi-Tenancy With Databases

// Option 1: Database-per-tenant
const db = client.db('tenant_' + tenantId);
db.users.find({});
// Each tenant has completely isolated data

// Option 2: Shared collection with tenantId field
const db = client.db('saas_app');
db.users.find({ tenantId: tenantId });
// All tenants share one collection, filtered by tenantId

Collection Naming Best Practices

// Good collection names
db.users.find({});
db.order_items.find({});
db.productCategories.find({});

// Avoid these
db['$sales'].find({});        // $ prefix causes issues
db['system.logs'].find({});   // reserved prefix
db['my.collection'].find({}); // dot in name causes confusion

The WiredTiger Storage Engine

// Check current storage engine
db.serverStatus().storageEngine;
// { name: 'wiredTiger', supportsCommittedReads: true, ... }

// Collection stats show compression savings
db.orders.stats();
// storageSize: 2097152 (compressed on disk)
// size: 8388608 (uncompressed logical size)

Understanding the Oplog

// Inspect oplog in mongosh (replica set members only)
use local
db.oplog.rs.find().sort({ $natural: -1 }).limit(3);
// { op: 'i', ns: 'shop.orders', o: { _id: ..., ... } }
// op: 'i'=insert, 'u'=update, 'd'=delete, 'c'=command

Quick Check

Lesson Recap