06. Complex Transformations: $unwind, $lookup (Joins) & $facet (Multi-Facet Analytics) • MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger • Rishabh Ki Kaksha

MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger › 06. Complex Transformations: $unwind, $lookup (Joins) & $facet (Multi-Facet Analytics)

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Course Syllabus

Module 1: MongoDB Foundations, BSON & CRUD Mechanics

01. NoSQL vs SQL: CAP Theorem & Document-Oriented Architecture 02. BSON Data Types & Document Schema Design (Embedding vs Referencing) 03. High-Performance CRUD Operations, BulkWrite & Write Concerns 04. Schema Validation with JSON Schema ($jsonSchema)

Module 2: Advanced Aggregation Framework & Data Pipelines

05. Aggregation Pipeline Core: $match, $project, $group, $sort & $limit 06. Complex Transformations: $unwind, $lookup (Joins) & $facet (Multi-Facet Analytics) 07. Window Operators ($setWindowFields) & Accumulator Functions

Module 3: Indexing Strategies & Query Optimization

08. B-Tree Indexes in MongoDB: Single Field, Compound & Partial Indexes 09. Special Indexes: TTL (Time-To-Live), Geospatial (2dsphere) & Text Indexes 10. Query Profiling, explain("executionStats") & Index Covering Analysis

Module 4: Storage Engine Internals (WiredTiger) & ACID Transactions

11. WiredTiger Storage Engine Architecture: Cache, Compression & Checkpoints 12. Read Concerns (local, majority, linearizable) & Write Concerns (w:majority, j:true) 13. Multi-Document Distributed ACID Transactions with Sessions

Module 5: High Availability, Replica Sets, Sharding & Backups

14. Replica Sets Architecture: Primary Election, Oplog & Automatic Failover 15. Sharding Architecture: mongos Router, Config Servers & Shard Key Selection 16. Range vs Hash Sharding, Chunk Balancing & Jumbo Chunks Mitigation 17. Change Streams: Building Real-Time Event-Driven Pipelines with MongoDB 18. Production Backup Strategies: mongodump/mongorestore, Point-in-Time & Atlas CLI

Interactive Lesson Studio

# 06. Complex Transformations: $unwind, $lookup (Joins) & $facet (Multi-Facet Analytics)

> MongoDB Notes by Rishabh - Topic 06

---

### 1. `$unwind`: Array Elements Ko Independent Documents Banana 🌪️

Agar ek order document me 3 items ka array hai:
```javascript
// Input: { orderId: 101, items: ["Laptop", "Mouse", "Keyboard"] }

// After $unwind: "$items" -> 3 Documents ban jaate hain!
// { orderId: 101, items: "Laptop" }
// { orderId: 101, items: "Mouse" }
// { orderId: 101, items: "Keyboard" }
```

---

### 2. `$lookup`: Relational Left Outer JOINs 🔗

```javascript
await db.collection('orders').aggregate([
    {
        $lookup: {
            from: "users",           // Target collection
            localField: "userId",    // Field in orders
            foreignField: "_id",     // Field in users
            as: "customerDetails"    // Output array field name
        }
    },
    // customerDetails array me 1 item hai -> unwind to object:
    {
        $unwind: {
            path: "$customerDetails",
            preserveNullAndEmptyArrays: true // Left Outer Join safety!
        }
    }
]);
```

---

### 3. `$facet`: Multi-Faceted E-Commerce Search in 1 Single Query 👑

Amazon/Flipkart search page par ek hi click par:
1. Paginated Products
2. Price Range Histogram
3. Top Brand Counts

```javascript
await db.collection('products').aggregate([
    { $match: { category: "Laptops" } },
    {
        $facet: {
            // Sub-pipeline 1: Paginated Products
            "products": [
                { $sort: { price: 1 } },
                { $skip: 0 },
                { $limit: 20 }
            ],
            // Sub-pipeline 2: Brand Distribution
            "brands": [
                { $group: { _id: "$brand", count: { $sum: 1 } } }
            ],
            // Sub-pipeline 3: Price Buckets
            "priceStats": [
                {
                    $bucket: {
                        groupBy: "$price",
                        boundaries: [0, 30000, 60000, 100000, 200000],
                        default: "Other"
                    }
                }
            ]
        }
    }
]);
```

---

### 4. Master Comparison Matrix

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### 5. Interview Ke 3 Sawal Is Topic Se

1. **`$unwind` me `preserveNullAndEmptyArrays: true` kyu zaroori hota hai?** -> Agar kisi document ka array empty `[]` ya `null` ho, toh default `$unwind` us document ko discard kar deta hai; ye flag use retain karta hai.
2. **Correlated Subquery `$lookup` (Pipeline syntax) kab use hota hai?** -> Jab join condition me complex expressions ya secondary collection par pre-filtering/aggregation chalani ho (`let: { orderUserId: "$userId" }`).
3. **`$facet` use karte waqt 16MB document limit ka dhyan kyu rakhna padta hai?** -> Kyunki `$facet` ka output single top-level BSON document hota hai, isliye sub-pipelines me `$limit` lagana best practice hai.

---
**Next Topic:** 07. Window Operators ($setWindowFields) & Accumulator Functions

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