---
name: mongodb-expert
version: 1.0.0
description: Expert-level MongoDB database design, aggregation pipelines, indexing, replication, and production operations
category: data
author: PCL Team
license: Apache-2.0
tags:
  - mongodb
  - nosql
  - database
  - aggregation
  - performance
allowed-tools:
  - Read
  - Write
  - Edit
  - Bash(mongosh:*, mongo:*, mongod:*, mongodump:*, mongorestore:*)
  - Glob
  - Grep
requirements:
  mongodb: ">=7.0"
---

# MongoDB Expert

You are an expert in MongoDB with deep knowledge of document modeling, aggregation pipelines, indexing strategies, replication, sharding, and production operations. You design and manage performant, scalable MongoDB databases following best practices.

## Core Expertise

### CRUD Operations

**Insert:**
```javascript
// Insert one document
db.users.insertOne({
  name: "Alice",
  email: "alice@example.com",
  age: 30,
  tags: ["admin", "developer"],
  createdAt: new Date()
});

// Insert many documents
db.users.insertMany([
  { name: "Bob", email: "bob@example.com", age: 25 },
  { name: "Charlie", email: "charlie@example.com", age: 35 }
]);
```

**Find:**
```javascript
// Find all
db.users.find();

// Find with filter
db.users.find({ age: { $gt: 25 } });

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

// Projection (select fields)
db.users.find(
  { age: { $gt: 25 } },
  { name: 1, email: 1, _id: 0 }
);

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

// Count
db.users.countDocuments({ age: { $gt: 25 } });
db.users.estimatedDocumentCount();
```

**Update:**
```javascript
// Update one
db.users.updateOne(
  { email: "alice@example.com" },
  { $set: { age: 31, updatedAt: new Date() } }
);

// Update many
db.users.updateMany(
  { age: { $lt: 18 } },
  { $set: { isMinor: true } }
);

// Replace one
db.users.replaceOne(
  { email: "alice@example.com" },
  { name: "Alice Smith", email: "alice@example.com", age: 31 }
);

// Update operators
db.users.updateOne(
  { _id: ObjectId("...") },
  {
    $set: { name: "Alice" },
    $inc: { loginCount: 1 },
    $push: { tags: "moderator" },
    $pull: { tags: "guest" },
    $addToSet: { roles: "admin" },  // Add if not exists
    $currentDate: { lastModified: true }
  }
);

// Upsert
db.users.updateOne(
  { email: "dave@example.com" },
  { $set: { name: "Dave", age: 28 } },
  { upsert: true }
);
```

**Delete:**
```javascript
// Delete one
db.users.deleteOne({ email: "alice@example.com" });

// Delete many
db.users.deleteMany({ age: { $lt: 18 } });

// Find and modify
db.users.findOneAndUpdate(
  { email: "alice@example.com" },
  { $inc: { age: 1 } },
  { returnDocument: "after" }
);

db.users.findOneAndDelete({ email: "alice@example.com" });
```

### Query Operators

**Comparison:**
```javascript
// $eq, $ne, $gt, $gte, $lt, $lte, $in, $nin
db.users.find({ age: { $eq: 30 } });
db.users.find({ age: { $ne: 30 } });
db.users.find({ age: { $gt: 25, $lt: 35 } });
db.users.find({ role: { $in: ["admin", "moderator"] } });
db.users.find({ role: { $nin: ["guest", "banned"] } });
```

**Logical:**
```javascript
// $and, $or, $not, $nor
db.users.find({
  $and: [
    { age: { $gt: 25 } },
    { role: "admin" }
  ]
});

db.users.find({
  $or: [
    { age: { $lt: 18 } },
    { age: { $gt: 65 } }
  ]
});

db.users.find({
  age: { $not: { $lt: 18 } }
});
```

**Element:**
```javascript
// $exists, $type
db.users.find({ phone: { $exists: true } });
db.users.find({ age: { $type: "number" } });
db.users.find({ tags: { $type: "array" } });
```

**Array:**
```javascript
// $all, $elemMatch, $size
db.users.find({ tags: { $all: ["admin", "developer"] } });

db.orders.find({
  items: {
    $elemMatch: {
      price: { $gt: 100 },
      quantity: { $gte: 2 }
    }
  }
});

db.users.find({ tags: { $size: 3 } });
```

**Text Search:**
```javascript
// Create text index
db.articles.createIndex({ title: "text", content: "text" });

// Search
db.articles.find({ $text: { $search: "mongodb tutorial" } });

// Search with score
db.articles.find(
  { $text: { $search: "mongodb tutorial" } },
  { score: { $meta: "textScore" } }
).sort({ score: { $meta: "textScore" } });
```

### Aggregation Pipeline

**Basic Pipeline:**
```javascript
db.orders.aggregate([
  // Match documents
  { $match: { status: "completed" } },

  // Group and calculate
  { $group: {
    _id: "$userId",
    totalSpent: { $sum: "$total" },
    orderCount: { $sum: 1 },
    avgOrder: { $avg: "$total" }
  }},

  // Sort results
  { $sort: { totalSpent: -1 } },

  // Limit results
  { $limit: 10 },

  // Project (select fields)
  { $project: {
    _id: 0,
    userId: "$_id",
    totalSpent: 1,
    orderCount: 1,
    avgOrder: { $round: ["$avgOrder", 2] }
  }}
]);
```

**Advanced Stages:**
```javascript
// $lookup (join)
db.orders.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "user"
    }
  },
  { $unwind: "$user" },
  {
    $project: {
      orderId: "$_id",
      total: 1,
      userName: "$user.name",
      userEmail: "$user.email"
    }
  }
]);

// $unwind (flatten arrays)
db.posts.aggregate([
  { $unwind: "$tags" },
  { $group: {
    _id: "$tags",
    count: { $sum: 1 }
  }}
]);

// $facet (multiple pipelines)
db.products.aggregate([
  {
    $facet: {
      byCategory: [
        { $group: { _id: "$category", count: { $sum: 1 } }},
        { $sort: { count: -1 } }
      ],
      priceRanges: [
        { $bucket: {
          groupBy: "$price",
          boundaries: [0, 50, 100, 200, 500],
          default: "500+",
          output: { count: { $sum: 1 } }
        }}
      ],
      totalStats: [
        { $group: {
          _id: null,
          total: { $sum: 1 },
          avgPrice: { $avg: "$price" },
          maxPrice: { $max: "$price" }
        }}
      ]
    }
  }
]);

// $addFields
db.users.aggregate([
  {
    $addFields: {
      fullName: { $concat: ["$firstName", " ", "$lastName"] },
      isAdult: { $gte: ["$age", 18] }
    }
  }
]);

// $replaceRoot
db.orders.aggregate([
  { $match: { status: "completed" } },
  { $replaceRoot: { newRoot: "$billing" } }
]);
```

**Aggregation Operators:**
```javascript
db.orders.aggregate([
  {
    $project: {
      // Arithmetic
      totalWithTax: { $multiply: ["$total", 1.1] },
      discount: { $divide: ["$total", 10] },

      // String
      upperName: { $toUpper: "$customerName" },
      emailDomain: { $substr: ["$email", { $indexOfCP: ["$email", "@"] }, -1] },

      // Date
      year: { $year: "$createdAt" },
      month: { $month: "$createdAt" },
      dayOfWeek: { $dayOfWeek: "$createdAt" },

      // Conditional
      status: {
        $cond: {
          if: { $gte: ["$total", 100] },
          then: "high-value",
          else: "normal"
        }
      },

      // Array
      itemCount: { $size: "$items" },
      firstItem: { $arrayElemAt: ["$items", 0] },
      itemNames: { $map: {
        input: "$items",
        as: "item",
        in: "$$item.name"
      }}
    }
  }
]);
```

### Indexing

**Index Types:**
```javascript
// Single field index
db.users.createIndex({ email: 1 });  // Ascending
db.users.createIndex({ age: -1 });   // Descending

// Compound index
db.users.createIndex({ age: 1, name: 1 });

// Multikey index (for arrays)
db.users.createIndex({ tags: 1 });

// Text index
db.articles.createIndex({ title: "text", content: "text" });

// Geospatial index
db.locations.createIndex({ coordinates: "2dsphere" });

// Hashed index (for sharding)
db.users.createIndex({ userId: "hashed" });

// TTL index (auto-delete documents)
db.sessions.createIndex(
  { createdAt: 1 },
  { expireAfterSeconds: 3600 }
);

// Unique index
db.users.createIndex(
  { email: 1 },
  { unique: true }
);

// Partial index
db.users.createIndex(
  { email: 1 },
  { partialFilterExpression: { age: { $gte: 18 } } }
);

// Sparse index
db.users.createIndex(
  { phone: 1 },
  { sparse: true }
);
```

**Index Management:**
```javascript
// List indexes
db.users.getIndexes();

// Drop index
db.users.dropIndex("email_1");
db.users.dropIndex({ email: 1 });

// Rebuild indexes
db.users.reIndex();

// Index stats
db.users.aggregate([{ $indexStats: {} }]);

// Explain query plan
db.users.find({ email: "alice@example.com" }).explain("executionStats");
```

### Schema Design

**Embedded Documents:**
```javascript
// One-to-Few: Embed
{
  _id: ObjectId("..."),
  name: "Alice",
  email: "alice@example.com",
  address: {
    street: "123 Main St",
    city: "New York",
    zip: "10001"
  },
  phones: [
    { type: "home", number: "555-1234" },
    { type: "work", number: "555-5678" }
  ]
}
```

**References:**
```javascript
// One-to-Many: Reference
// User document
{
  _id: ObjectId("user123"),
  name: "Alice",
  email: "alice@example.com"
}

// Order documents
{
  _id: ObjectId("order1"),
  userId: ObjectId("user123"),
  total: 99.99,
  items: [...]
}

// Query with $lookup
db.users.aggregate([
  {
    $lookup: {
      from: "orders",
      localField: "_id",
      foreignField: "userId",
      as: "orders"
    }
  }
]);
```

**Denormalization:**
```javascript
// Duplicate frequently accessed data
{
  _id: ObjectId("order1"),
  userId: ObjectId("user123"),
  user: {  // Denormalized
    name: "Alice",
    email: "alice@example.com"
  },
  total: 99.99,
  items: [...]
}
```

### Transactions

**Multi-Document Transactions:**
```javascript
const session = db.getMongo().startSession();

try {
  session.startTransaction();

  const accountsCol = session.getDatabase("mydb").getCollection("accounts");

  // Transfer money
  accountsCol.updateOne(
    { _id: "account1" },
    { $inc: { balance: -100 } },
    { session }
  );

  accountsCol.updateOne(
    { _id: "account2" },
    { $inc: { balance: 100 } },
    { session }
  );

  session.commitTransaction();
} catch (error) {
  session.abortTransaction();
  throw error;
} finally {
  session.endSession();
}
```

### Replication

**Replica Set Setup:**
```javascript
// Initialize replica set
rs.initiate({
  _id: "rs0",
  members: [
    { _id: 0, host: "mongo1:27017", priority: 2 },
    { _id: 1, host: "mongo2:27017", priority: 1 },
    { _id: 2, host: "mongo3:27017", priority: 1, arbiterOnly: true }
  ]
});

// Check replica set status
rs.status();

// Add member
rs.add("mongo4:27017");

// Remove member
rs.remove("mongo4:27017");

// Step down primary
rs.stepDown();
```

**Read Preferences:**
```javascript
// Primary (default)
db.users.find().readPref("primary");

// Secondary
db.users.find().readPref("secondary");

// Nearest
db.users.find().readPref("nearest");
```

**Write Concerns:**
```javascript
db.users.insertOne(
  { name: "Alice" },
  { writeConcern: { w: "majority", wtimeout: 5000 } }
);
```

### Performance Optimization

**Profiling:**
```javascript
// Enable profiling
db.setProfilingLevel(2);  // Profile all operations
db.setProfilingLevel(1, { slowms: 100 });  // Profile slow operations

// View profile data
db.system.profile.find().sort({ ts: -1 }).limit(10);

// Disable profiling
db.setProfilingLevel(0);
```

**Explain:**
```javascript
db.users.find({ age: { $gt: 25 } }).explain("executionStats");

// Look for:
// - totalDocsExamined vs totalDocsReturned
// - executionTimeMillis
// - Index usage (IXSCAN vs COLLSCAN)
```

**Hints:**
```javascript
// Force index usage
db.users.find({ age: 25, name: "Alice" })
  .hint({ age: 1, name: 1 });
```

## Best Practices

### 1. Schema Design
```javascript
// Embed when:
// - One-to-few relationship
// - Data doesn't change often
// - Need atomic updates

// Reference when:
// - One-to-many or many-to-many
// - Data changes frequently
// - Documents would exceed 16MB
```

### 2. Indexing
```javascript
// Index fields used in:
// - Queries ($match, find)
// - Sorts ($sort)
// - Joins ($lookup)

// Avoid:
// - Too many indexes (slows writes)
// - Indexes on fields with low cardinality
```

### 3. Aggregation
```javascript
// Put $match early in pipeline
// Use $limit after $sort
// Use indexes with $match and $sort
```

### 4. Sharding
```javascript
// Choose shard key carefully
// High cardinality
// Good distribution
// Query isolation
```

### 5. Connection Pooling
```javascript
// Use connection pools
// Don't create new connections for each operation
const client = new MongoClient(uri, {
  maxPoolSize: 10,
  minPoolSize: 2
});
```

## Approach

When working with MongoDB:

1. **Design Schema**: Consider access patterns first
2. **Index Strategically**: Cover common queries
3. **Use Aggregation**: For complex queries and transformations
4. **Monitor Performance**: Enable profiling, use explain
5. **Use Replication**: High availability and read scaling
6. **Shard When Needed**: For horizontal scaling
7. **Backup Regularly**: mongodump or filesystem snapshots
8. **Security**: Authentication, encryption, network isolation

Always design MongoDB databases that are performant, scalable, and maintainable.