{
  "_id": "507f1f77bcf86cd799439011",
  "title": "What is MongoDB?",
  "author": "Rahul Sharma",
  "tags": ["database", "NoSQL", "MongoDB"],
  "views": 15420,
  "published": true,
  "publishedAt": "2026-01-15T10:30:00Z",
  "content": {
    "introduction": "MongoDB is...",
    "sections": 8
  }
}

Database: futuretechzone
  ├── users (collection)
  │     ├── {_id, name, email, role, ...}
  │     └── {_id, name, email, plan, company, ...}
  ├── articles (collection)
  │     ├── {_id, title, content, tags, ...}
  │     └── {_id, title, videoUrl, duration, ...}
  └── comments (collection)
        └── {_id, articleId, userId, text, likes, ...}

ObjectId("507f1f77bcf86cd799439011")
//         |---------||----||----|-------|
//         timestamp  machine  pid  counter

// These data types exist in BSON but not in standard JSON
{
  "_id": ObjectId("507f..."),           // ObjectId
  "createdAt": new Date(),              // Date (timestamp)
  "price": NumberDecimal("9.99"),       // High-precision decimal
  "profilePhoto": BinData(0, "abc..."), // Binary data
  "isDeleted": false,                   // Boolean
  "tags": ["node", "mongodb"],          // Array
  "nested": { "key": "value" }          // Embedded document
}

// Insert one document
db.users.insertOne({
    name: "Rahul Sharma",
    email: "rahul@email.com",
    age: 25,
    role: "admin",
    createdAt: new Date()
});

// Insert multiple documents at once
db.users.insertMany([
    { name: "Priya Patel", email: "priya@email.com", age: 28 },
    { name: "Arjun Singh", email: "arjun@email.com", age: 32 },
    { name: "Meera Nair", email: "meera@email.com", age: 24 }
]);

// Find all documents in a collection
db.users.find();

// Find with filter
db.users.find({ role: "admin" });

// Find with multiple conditions
db.users.find({ age: { $gte: 25 }, role: "admin" });

// Find one document
db.users.findOne({ email: "rahul@email.com" });

// Find with projection (select specific fields)
db.users.find({}, { name: 1, email: 1, _id: 0 });

// Sort, limit, skip
db.users.find().sort({ age: -1 }).limit(10).skip(20);

// Update one document
db.users.updateOne(
    { email: "rahul@email.com" },     // Filter
    { $set: { role: "superadmin" } }  // Update
);

// Update multiple documents
db.users.updateMany(
    { age: { $lt: 18 } },
    { $set: { category: "minor" } }
);

// Increment a field
db.articles.updateOne(
    { _id: articleId },
    { $inc: { views: 1 } }
);

// Add to an array
db.users.updateOne(
    { email: "rahul@email.com" },
    { $push: { tags: "premium" } }
);

// Delete one document
db.users.deleteOne({ email: "spam@example.com" });

// Delete multiple documents
db.users.deleteMany({ isDeleted: true });

// Delete all documents in a collection
db.users.deleteMany({});

// Create a single field index
db.users.createIndex({ email: 1 });   // 1 = ascending, -1 = descending

// Create a compound index (multiple fields)
db.articles.createIndex({ author: 1, publishedAt: -1 });

// Create a unique index (prevents duplicates)
db.users.createIndex({ email: 1 }, { unique: true });

// Create a text index (for full-text search)
db.articles.createIndex({ title: "text", content: "text" });

// Create a TTL index (auto-delete after time)
db.sessions.createIndex(
    { createdAt: 1 },
    { expireAfterSeconds: 3600 } // Delete 1 hour after createdAt
);

// View all indexes on a collection
db.users.getIndexes();

// See how MongoDB executes a query
db.users.find({ email: "rahul@email.com" }).explain("executionStats");

// Look for:
// "IXSCAN" = Index scan (fast ✅)
// "COLLSCAN" = Collection scan (slow ❌ — add an index)

// Aggregation pipeline example:
// Find top 5 authors by total article views in the last 30 days

db.articles.aggregate([
    // Stage 1: Filter — only articles from last 30 days
    {
        $match: {
            publishedAt: { $gte: new Date(Date.now() - 30 * 24 * 60 * 60 * 1000) }
        }
    },
    // Stage 2: Group — sum views by author
    {
        $group: {
            _id: "$author",
            totalViews: { $sum: "$views" },
            articleCount: { $count: {} }
        }
    },
    // Stage 3: Sort — highest views first
    {
        $sort: { totalViews: -1 }
    },
    // Stage 4: Limit — top 5 only
    {
        $limit: 5
    },
    // Stage 5: Project — rename fields for output
    {
        $project: {
            author: "$_id",
            totalViews: 1,
            articleCount: 1,
            _id: 0
        }
    }
]);

-- SQL equivalent of the above
SELECT author, SUM(views) as totalViews, COUNT(*) as articleCount
FROM articles
WHERE publishedAt >= DATE_SUB(NOW(), INTERVAL 30 DAY)
GROUP BY author
ORDER BY totalViews DESC
LIMIT 5;