mongodb-expert

What this skill does

# 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: "[email protected]",
  age: 30,
  tags: ["admin", "developer"],
  createdAt: new Date()
});

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

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

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

// Find one
db.users.findOne({ email: "[email protected]" });

// 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: "[email protected]" },
  { $set: { age: 31, updatedAt: new Date() } }
);

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

// Replace one
db.users.replaceOne(
  { email: "[email protected]" },
  { name: "Alice Smith", email: "[email protected]", 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: "[email protected]" },
  { $set: { name: "Dave", age: 28 } },
  { upsert: true }
);
```

**Delete:**
```javascript
// Delete one
db.users.deleteOne({ email: "[email protected]" });

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

// Find and modify
db.users.findOneAndUpdate(
  { email: "[email protected]" },
  { $inc: { age: 1 } },
  { returnDocument: "after" }
);

db.users.findOneAndDelete({ email: "[email protected]" });
```

### 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: "[email protected]" }).explain("executionStats");
```

### Schema Design

**Embedded Documents:**
```javascript
// One-to-Few: Embed
{
  _id: ObjectId("..."),
  name: "Alice",
  email: "[email protected]",
  address: