MongoDB Aggregation

1. What is MongoDB Aggregation?

MongoDB aggregation is a way to process and transform data inside the database.

Think of it like a pipeline:

Documents → Step 1 → Step 2 → Step 3 → Result

Each step modifies, filters, groups, or reshapes the data.

MongoDB uses the Aggregation Pipeline:

db.collection.aggregate([
  { stage1 },
  { stage2 },
  { stage3 }
])

2. Why Use Aggregation?

Aggregation helps you:

Filter data
Group documents
Calculate totals/averages
Join collections
Transform fields
Sort & paginate
Build analytics

Similar to:

SQL GROUP BY
SQL JOIN
SQL calculations

3. Sample Data

We'll use this sample data to demonstrate aggregation stages:

products.js

[
  {
    _id: 101,
    name: "iPhone 15",
    category: "Mobile",
    brand: "Apple",
    price: 85000,
    stock: 12,
    tags: ["electronics", "phone", "ios"],
    ratings: [5, 4, 5, 3, 4],
    createdAt: new Date("2025-01-10")
  },
  {
    _id: 102,
    name: "Samsung S24",
    category: "Mobile",
    brand: "Samsung",
    price: 72000,
    stock: 20,
    tags: ["android", "electronics"],
    ratings: [4, 4, 5],
    createdAt: new Date("2025-02-11")
  },
  {
    _id: 103,
    name: "MacBook Air",
    category: "Laptop",
    brand: "Apple",
    price: 120000,
    stock: 7,
    tags: ["laptop", "macos"],
    ratings: [5, 5, 5],
    createdAt: new Date("2024-09-01")
  },
  {
    _id: 104,
    name: "Boat Headphones",
    category: "Accessories",
    brand: "Boat",
    price: 2500,
    stock: 50,
    tags: ["audio", "music"],
    ratings: [3, 4, 4],
    createdAt: new Date("2025-03-14")
  },
  {
    _id: 105,
    name: "Gaming Mouse",
    category: "Accessories",
    brand: "Logitech",
    price: 1800,
    stock: 35,
    tags: ["gaming", "pc"],
    ratings: [5, 4, 4, 5],
    createdAt: new Date("2025-01-20")
  }
];

orders.js

[
  {
    _id: 1001,
    userId: 1,
    products: [
      {
        productId: 101,
        quantity: 1
      },
      {
        productId: 104,
        quantity: 2
      }
    ],
    totalAmount: 90000,
    status: "DELIVERED",
    paymentMethod: "UPI",
    shippingAddress: {
      city: "Bhagalpur",
      state: "Bihar"
    },
    orderDate: new Date("2025-04-01")
  },
  {
    _id: 1002,
    userId: 2,
    products: [
      {
        productId: 102,
        quantity: 1
      }
    ],
    totalAmount: 72000,
    status: "PENDING",
    paymentMethod: "Card",
    shippingAddress: {
      city: "Patna",
      state: "Bihar"
    },
    orderDate: new Date("2025-04-05")
  },
  {
    _id: 1003,
    userId: 3,
    products: [
      {
        productId: 103,
        quantity: 1
      },
      {
        productId: 105,
        quantity: 2
      }
    ],
    totalAmount: 123600,
    status: "DELIVERED",
    paymentMethod: "NetBanking",
    shippingAddress: {
      city: "Delhi",
      state: "Delhi"
    },
    orderDate: new Date("2025-04-10")
  },
  {
    _id: 1004,
    userId: 1,
    products: [
      {
        productId: 105,
        quantity: 3
      }
    ],
    totalAmount: 5400,
    status: "CANCELLED",
    paymentMethod: "COD",
    shippingAddress: {
      city: "Bhagalpur",
      state: "Bihar"
    },
    orderDate: new Date("2025-04-15")
  },
  {
    _id: 1011,
    userId: 2,
    products: [
      {
        productId: 101,
        quantity: 1
      },
      {
        productId: 104,
        quantity: 2
      }
    ],
    totalAmount: 90000,
    status: "DELIVERED",
    paymentMethod: "UPI",
    shippingAddress: {
      city: "Patna",
      state: "Bihar"
    },
    orderDate: new Date()
  },
  {
    _id: 1012,
    userId: 1,
    products: [
      {
        productId: 103,
        quantity: 1
      }
    ],
    totalAmount: 120000,
    status: "DELIVERED",
    paymentMethod: "Card",
    shippingAddress: {
      city: "Bhagalpur",
      state: "Bihar"
    },
    orderDate: new Date(Date.now() - 5 * 60 * 60 * 1000)
  },
  {
    _id: 1013,
    userId: 3,
    products: [
      {
        productId: 105,
        quantity: 3
      }
    ],
    totalAmount: 5400,
    status: "PENDING",
    paymentMethod: "COD",
    shippingAddress: {
      city: "Delhi",
      state: "Delhi"
    },
    orderDate: new Date(Date.now() - 10 * 60 * 60 * 1000)
  }
];

4. Most Important Aggregation Stages

4.1. `$match` : Filter Documents

Like SQL WHERE.

$match is used to:

Filter documents based on a condition.

1. Filter pending orders.

db.orders.aggregate([
  {
    $match: {
      status: "PENDING"
    }
  }
]);

Output:

[
  {
    _id: 1002,
    userId: 2,
    products: [ { productId: 102, quantity: 1 } ],
    totalAmount: 72000,
    status: 'PENDING',
    paymentMethod: 'Card',
    shippingAddress: { city: 'Patna', state: 'Bihar' },
    orderDate: ISODate('2025-04-05T00:00:00.000Z')
  },
  {
    _id: 1013,
    userId: 3,
    products: [ { productId: 105, quantity: 3 } ],
    totalAmount: 5400,
    status: 'PENDING',
    paymentMethod: 'COD',
    shippingAddress: { city: 'Delhi', state: 'Delhi' },
    orderDate: ISODate('2026-05-27T01:30:29.284Z')
  }
]

2. Filter all products whose price is greater than 10000.

db.products.aggregate([
  {
    $match: {
      price: {
        $gt: 10000
      }
    }
  }
]);

Output:

[
  {
    _id: 101,
    name: "iPhone 15",
    category: "Mobile",
    brand: "Apple",
    price: 85000,
    stock: 12,
    tags: ["electronics", "phone", "ios"],
    ratings: [5, 4, 5, 3, 4],
    createdAt: ISODate("2025-01-10T00:00:00.000Z")
  },
  {
    _id: 102,
    name: "Samsung S24",
    category: "Mobile",
    brand: "Samsung",
    price: 72000,
    stock: 20,
    tags: ["android", "electronics"],
    ratings: [4, 4, 5],
    createdAt: ISODate("2025-02-11T00:00:00.000Z")
  },
  {
    _id: 103,
    name: "MacBook Air",
    category: "Laptop",
    brand: "Apple",
    price: 120000,
    stock: 7,
    tags: ["laptop", "macos"],
    ratings: [5, 5, 5],
    createdAt: ISODate("2024-09-01T00:00:00.000Z")
  }
];

4.2 `$group` : Group Documents

Like SQL GROUP BY.

$group is used to:

Group documents by a common field and apply an aggregation operation to each group.

_id means: Group by this field Not the actual MongoDB document ID.

1. Count how many products are in each category.

db.products.aggregate([
  {
    $group: {
      _id: "$category",
      totalProducts: {
        $sum: 1
      },
      products: {
        $push: {
          _id: "$_id",
          name: "$name",
          price: "$price"
        }
      }
    }
  }
]);

Output:

[
  {
    _id: "Accessories",
    totalProducts: 2,
    products: [
      {
        _id: 104,
        name: "Boat Headphones",
        price: 2500
      },
      {
        _id: 105,
        name: "Gaming Mouse",
        price: 1800
      }
    ]
  },
  {
    _id: "Laptop",
    totalProducts: 1,
    products: [
      {
        _id: 103,
        name: "MacBook Air",
        price: 120000
      }
    ]
  },
  {
    _id: "Mobile",
    totalProducts: 2,
    products: [
      {
        _id: 101,
        name: "iPhone 15",
        price: 85000
      },
      {
        _id: 102,
        name: "Samsung S24",
        price: 72000
      }
    ]
  }
];

2. Count total stock & products per category.

db.products.aggregate([
  {
    $group: {
      _id: "$category",
      totalProducts: {
        $sum: 1
      },
      totalStock: {
        $sum: "$stock"
      }
    }
  }
]);

Output:

[
  {
    _id: "Mobile",
    totalProducts: 2,
    totalStock: 32
  },
  {
    _id: "Laptop",
    totalProducts: 1,
    totalStock: 7
  },
  {
    _id: "Accessories",
    totalProducts: 2,
    totalStock: 85
  }
];

3. Count total price per category.

db.products.aggregate([
  {
    $group: {
      _id: "$category",
      totalPrice: {
        $sum: "$price"
      }
    }
  }
]);

Output:

[
  { _id: "Mobile", totalPrice: 157000 },
  { _id: "Laptop", totalPrice: 120000 },
  { _id: "Accessories", totalPrice: 4300 }
];

4.3 `$project` : Select/Transform Fields

$project is used to:

Select or transform fields in the output documents.

db.products.aggregate([
  {
    $match: {
      brand: "Apple"
    }
  },
  {
    $project: {
      name: 1,
      price: 1,
      category: 1,
      brand: 1
    }
  }
]);

Output:

[
  {
    _id: 101,
    name: 'iPhone 15',
    category: 'Mobile',
    brand: 'Apple',
    price: 85000
  },
  {
    _id: 103,
    name: 'MacBook Air',
    category: 'Laptop',
    brand: 'Apple',
    price: 120000
  }
]

4.4 `$sort` : Sort Documents

$sort is used to:

Sort documents in ascending or descending order.

db.products.aggregate([
  {
    $match: {
      brand: "Apple"
    }
  },
  {
    $project: {
      name: 1,
      price: 1,
      category: 1,
      brand: 1
    }
  },
  {
    $sort: {
      price: 1
    }
  }
]);

1 : sort by ascending order.
-1 : sort by descending order.

Output:

[
  {
    _id: 101,
    name: 'iPhone 15',
    category: 'Mobile',
    brand: 'Apple',
    price: 85000
  },
  {
    _id: 103,
    name: 'MacBook Air',
    category: 'Laptop',
    brand: 'Apple',
    price: 120000
  }
]

4.5 `$limit` : Limit Documents

$limit is used to:

Limit the number of documents returned.

db.products.aggregate([
  {
    $limit: 3
  }
]);

Output:

[
  {
    _id: 101,
    name: "iPhone 15",
    category: "Mobile",
    brand: "Apple",
    price: 85000,
    stock: 12,
    tags: ["electronics", "phone", "ios"],
    ratings: [5, 4, 5, 3, 4],
    createdAt: ISODate("2025-01-10T00:00:00.000Z")
  },
  {
    _id: 102,
    name: "Samsung S24",
    category: "Mobile",
    brand: "Samsung",
    price: 72000,
    stock: 20,
    tags: ["android", "electronics"],
    ratings: [4, 4, 5],
    createdAt: ISODate("2025-02-11T00:00:00.000Z")
  },
  {
    _id: 103,
    name: "MacBook Air",
    category: "Laptop",
    brand: "Apple",
    price: 120000,
    stock: 7,
    tags: ["laptop", "macos"],
    ratings: [5, 5, 5],
    createdAt: ISODate("2024-09-01T00:00:00.000Z")
  }
];

4.6 `$skip` : Skip Documents

$skip is used to:

Skip documents before returning the first document. Useful for pagination.

db.products.aggregate([
  {
    $skip: 2
  }
]);

Output:

[
  {
    _id: 103,
    name: "MacBook Air",
    category: "Laptop",
    brand: "Apple",
    price: 120000,
    stock: 7,
    tags: ["laptop", "macos"],
    ratings: [5, 5, 5],
    createdAt: ISODate("2024-09-01T00:00:00.000Z")
  },
  {
    _id: 104,
    name: "Boat Headphones",
    category: "Accessories",
    brand: "Boat",
    price: 2500,
    stock: 50,
    tags: ["audio", "music"],
    ratings: [3, 4, 4],
    createdAt: ISODate("2025-03-14T00:00:00.000Z")
  },
  {
    _id: 105,
    name: "Gaming Mouse",
    category: "Accessories",
    brand: "Logitech",
    price: 1800,
    stock: 35,
    tags: ["gaming", "pc"],
    ratings: [5, 4, 4, 5],
    createdAt: ISODate("2025-01-20T00:00:00.000Z")
  }
];

4.7 `$lookup` : Join Collections

Like SQL JOIN.

$lookup is used to:

Join data from other collections.

1. Find two orders with products details.

db.orders.aggregate([
  {
    $lookup: {
      from: "products",
      localField: "products.productId",
      foreignField: "_id",
      as: "productDetails"
    }
  },
  {
    $limit: 2
  }
]);

Output:

[
  {
    _id: 1001,
    userId: 1,
    products: [
      { productId: 101, quantity: 1 },
      { productId: 104, quantity: 2 }
    ],
    totalAmount: 90000,
    status: "DELIVERED",
    paymentMethod: "UPI",
    shippingAddress: { city: "Bhagalpur", state: "Bihar" },
    orderDate: ISODate("2025-04-01T00:00:00.000Z"),
    productDetails: [
      {
        _id: 104,
        name: "Boat Headphones",
        category: "Accessories",
        brand: "Boat",
        price: 2500,
        stock: 50,
        tags: ["audio", "music"],
        ratings: [3, 4, 4],
        createdAt: ISODate("2025-03-14T00:00:00.000Z")
      },
      {
        _id: 101,
        name: "iPhone 15",
        category: "Mobile",
        brand: "Apple",
        price: 85000,
        stock: 12,
        tags: ["electronics", "phone", "ios"],
        ratings: [5, 4, 5, 3, 4],
        createdAt: ISODate("2025-01-10T00:00:00.000Z")
      }
    ]
  },
  {
    _id: 1002,
    userId: 2,
    products: [{ productId: 102, quantity: 1 }],
    totalAmount: 72000,
    status: "PENDING",
    paymentMethod: "Card",
    shippingAddress: { city: "Patna", state: "Bihar" },
    orderDate: ISODate("2025-04-05T00:00:00.000Z"),
    productDetails: [
      {
        _id: 102,
        name: "Samsung S24",
        category: "Mobile",
        brand: "Samsung",
        price: 72000,
        stock: 20,
        tags: ["android", "electronics"],
        ratings: [4, 4, 5],
        createdAt: ISODate("2025-02-11T00:00:00.000Z")
      }
    ]
  }
];

2. Find orders with products details(_id, name, price, brand).

db.orders.aggregate([
  {
    $lookup: {
      from: "products",
      localField: "products.productId",
      foreignField: "_id",
      pipeline: [
        {
          $project: {
            name: 1,
            price: 1,
            brand: 1
          }
        }
      ],
      as: "productDetails"
    }
  },
  {
    $limit: 2
  }
]);

Output:

[
  {
    _id: 1001,
    userId: 1,
    products: [
      {
        productId: 101,
        quantity: 1
      },
      { 
        productId: 104,
        quantity: 2 
      }
    ],
    totalAmount: 90000,
    status: "DELIVERED",
    paymentMethod: "UPI",
    shippingAddress: {
      city: "Bhagalpur",
      state: "Bihar"
    },
    orderDate: ISODate("2025-04-01T00:00:00.000Z"),
    productDetails: [
      {
        _id: 101,
        name: "iPhone 15",
        brand: "Apple",
        price: 85000
      },
      {
        _id: 104,
        name: "Boat Headphones",
        brand: "Boat",
        price: 2500
      }
    ]
  },
  {
    _id: 1002,
    userId: 2,
    products: [
      {
        productId: 102,
        quantity: 1
      }
    ],
    totalAmount: 72000,
    status: "PENDING",
    paymentMethod: "Card",
    shippingAddress: {
      city: "Patna",
      state: "Bihar"
    },
    orderDate: ISODate("2025-04-05T00:00:00.000Z"),
    productDetails: [
      {
        _id: 102,
        name: "Samsung S24",
        brand: "Samsung",
        price: 72000
      }
    ]
  }
];

4.8 `$unwind` : Unwind Arrays

$unwind is used to:

Break an array into separate documents.

1. Unwind ratings array.

db.products.aggregate([
  {
    $unwind: "$ratings"
  },
  {
    $project: {
      name: 1,
      ratings: 1
    }
  },
  {
    $limit: 4
  }
]);

Output:

[
  {
    _id: 101,
    name: "iPhone 15",
    ratings: 5
  },
  {
    _id: 101,
    name: "iPhone 15",
    ratings: 4
  },
  {
    _id: 101,
    name: "iPhone 15",
    ratings: 5
  },
  {
    _id: 101,
    name: "iPhone 15",
    ratings: 3
  }
];

2. Count How Many Times Each Rating Appears

db.products.aggregate([
  {
    $unwind: "$ratings"
  },
  {
    $group: {
      _id: "$ratings",
      total: {
        $sum: 1
      }
    }
  },
  {
    $sort: {
      _id: -1
    }
  }
]);

Output:

[
  { _id: 5, total: 8 },
  { _id: 4, total: 8 },
  { _id: 3, total: 2 }
];

5. Aggregation Pipeline Example

Find the total revenue generated by each category of products in the last 24 hours.

db.orders.aggregate([
  {
    $match: {
      // last 24 hours
      orderDate: {
        $gt: new Date(Date.now() - 24 * 60 * 60 * 1000)
      }
    }
  },
  {
    $project: {
      products: 1
    }
  },
  {
    $unwind: "$products"
  },
  {
    $lookup: {
      from: "products",
      localField: "products.productId",
      foreignField: "_id",
    
      as: "productDetails"
    }
  },
  {
    $unwind: "$productDetails"
  },
  {
    $group: {
      _id: "$productDetails.category",
      totalRevenue: {
        $sum: {
          $multiply: ["$products.quantity", "$productDetails.price"]
        }
      }
    }
  }
]);

Output:

[
  { _id: "Accessories", totalRevenue: 10400 },
  { _id: "Laptop", totalRevenue: 120000 },
  { _id: "Mobile", totalRevenue: 85000 }
];

6. MongoDB Aggregation Stages & Their Uses

Stage	Use
$match	Filter documents
$group	Group documents and calculate values
$project	Select, remove, rename, or transform fields
$sort	Sort documents
$limit	Limit number of documents
$skip	Skip documents
$unwind	Split array into multiple documents
$lookup	Join another collection
$addFields	Add new fields
$set	Same as $addFields
$unset	Remove fields
$count	Count total documents
$sample	Get random documents
$replaceRoot	Replace entire document
$replaceWith	Same as $replaceRoot
$sortByCount	Group, count, and sort documents
$unionWith	Combine multiple collections
$out	Save aggregation result into collection
$geoNear	Find nearby locations

[ { _id: 101, name: "iPhone 15", category: "Mobile", brand: "Apple", price: 85000, stock: 12, tags: ["electronics", "phone", "ios"], ratings: [5, 4, 5, 3, 4], createdAt: new Date("2025-01-10") }, { _id: 102, name: "Samsung S24", category: "Mobile", brand: "Samsung", price: 72000, stock: 20, tags: ["android", "electronics"], ratings: [4, 4, 5], createdAt: new Date("2025-02-11") }, { _id: 103, name: "MacBook Air", category: "Laptop", brand: "Apple", price: 120000, stock: 7, tags: ["laptop", "macos"], ratings: [5, 5, 5], createdAt: new Date("2024-09-01") }, { _id: 104, name: "Boat Headphones", category: "Accessories", brand: "Boat", price: 2500, stock: 50, tags: ["audio", "music"], ratings: [3, 4, 4], createdAt: new Date("2025-03-14") }, { _id: 105, name: "Gaming Mouse", category: "Accessories", brand: "Logitech", price: 1800, stock: 35, tags: ["gaming", "pc"], ratings: [5, 4, 4, 5], createdAt: new Date("2025-01-20") } ];

[ { _id: 1001, userId: 1, products: [ { productId: 101, quantity: 1 }, { productId: 104, quantity: 2 } ], totalAmount: 90000, status: "DELIVERED", paymentMethod: "UPI", shippingAddress: { city: "Bhagalpur", state: "Bihar" }, orderDate: new Date("2025-04-01") }, { _id: 1002, userId: 2, products: [ { productId: 102, quantity: 1 } ], totalAmount: 72000, status: "PENDING", paymentMethod: "Card", shippingAddress: { city: "Patna", state: "Bihar" }, orderDate: new Date("2025-04-05") }, { _id: 1003, userId: 3, products: [ { productId: 103, quantity: 1 }, { productId: 105, quantity: 2 } ], totalAmount: 123600, status: "DELIVERED", paymentMethod: "NetBanking", shippingAddress: { city: "Delhi", state: "Delhi" }, orderDate: new Date("2025-04-10") }, { _id: 1004, userId: 1, products: [ { productId: 105, quantity: 3 } ], totalAmount: 5400, status: "CANCELLED", paymentMethod: "COD", shippingAddress: { city: "Bhagalpur", state: "Bihar" }, orderDate: new Date("2025-04-15") }, { _id: 1011, userId: 2, products: [ { productId: 101, quantity: 1 }, { productId: 104, quantity: 2 } ], totalAmount: 90000, status: "DELIVERED", paymentMethod: "UPI", shippingAddress: { city: "Patna", state: "Bihar" }, orderDate: new Date() }, { _id: 1012, userId: 1, products: [ { productId: 103, quantity: 1 } ], totalAmount: 120000, status: "DELIVERED", paymentMethod: "Card", shippingAddress: { city: "Bhagalpur", state: "Bihar" }, orderDate: new Date(Date.now() - 5 * 60 * 60 * 1000) }, { _id: 1013, userId: 3, products: [ { productId: 105, quantity: 3 } ], totalAmount: 5400, status: "PENDING", paymentMethod: "COD", shippingAddress: { city: "Delhi", state: "Delhi" }, orderDate: new Date(Date.now() - 10 * 60 * 60 * 1000) } ];

[ { _id: 1002, userId: 2, products: [ { productId: 102, quantity: 1 } ], totalAmount: 72000, status: 'PENDING', paymentMethod: 'Card', shippingAddress: { city: 'Patna', state: 'Bihar' }, orderDate: ISODate('2025-04-05T00:00:00.000Z') }, { _id: 1013, userId: 3, products: [ { productId: 105, quantity: 3 } ], totalAmount: 5400, status: 'PENDING', paymentMethod: 'COD', shippingAddress: { city: 'Delhi', state: 'Delhi' }, orderDate: ISODate('2026-05-27T01:30:29.284Z') } ]

[ { _id: 101, name: "iPhone 15", category: "Mobile", brand: "Apple", price: 85000, stock: 12, tags: ["electronics", "phone", "ios"], ratings: [5, 4, 5, 3, 4], createdAt: ISODate("2025-01-10T00:00:00.000Z") }, { _id: 102, name: "Samsung S24", category: "Mobile", brand: "Samsung", price: 72000, stock: 20, tags: ["android", "electronics"], ratings: [4, 4, 5], createdAt: ISODate("2025-02-11T00:00:00.000Z") }, { _id: 103, name: "MacBook Air", category: "Laptop", brand: "Apple", price: 120000, stock: 7, tags: ["laptop", "macos"], ratings: [5, 5, 5], createdAt: ISODate("2024-09-01T00:00:00.000Z") } ];

db.products.aggregate([ { $group: { _id: "$category", totalProducts: { $sum: 1 }, products: { $push: { _id: "$_id", name: "$name", price: "$price" } } } } ]);

[ { _id: "Accessories", totalProducts: 2, products: [ { _id: 104, name: "Boat Headphones", price: 2500 }, { _id: 105, name: "Gaming Mouse", price: 1800 } ] }, { _id: "Laptop", totalProducts: 1, products: [ { _id: 103, name: "MacBook Air", price: 120000 } ] }, { _id: "Mobile", totalProducts: 2, products: [ { _id: 101, name: "iPhone 15", price: 85000 }, { _id: 102, name: "Samsung S24", price: 72000 } ] } ];

[ { _id: "Mobile", totalProducts: 2, totalStock: 32 }, { _id: "Laptop", totalProducts: 1, totalStock: 7 }, { _id: "Accessories", totalProducts: 2, totalStock: 85 } ];

[ { _id: 101, name: 'iPhone 15', category: 'Mobile', brand: 'Apple', price: 85000 }, { _id: 103, name: 'MacBook Air', category: 'Laptop', brand: 'Apple', price: 120000 } ]

db.products.aggregate([ { $match: { brand: "Apple" } }, { $project: { name: 1, price: 1, category: 1, brand: 1 } }, { $sort: { price: 1 } } ]);

[ { _id: 101, name: 'iPhone 15', category: 'Mobile', brand: 'Apple', price: 85000 }, { _id: 103, name: 'MacBook Air', category: 'Laptop', brand: 'Apple', price: 120000 } ]

[ { _id: 103, name: "MacBook Air", category: "Laptop", brand: "Apple", price: 120000, stock: 7, tags: ["laptop", "macos"], ratings: [5, 5, 5], createdAt: ISODate("2024-09-01T00:00:00.000Z") }, { _id: 104, name: "Boat Headphones", category: "Accessories", brand: "Boat", price: 2500, stock: 50, tags: ["audio", "music"], ratings: [3, 4, 4], createdAt: ISODate("2025-03-14T00:00:00.000Z") }, { _id: 105, name: "Gaming Mouse", category: "Accessories", brand: "Logitech", price: 1800, stock: 35, tags: ["gaming", "pc"], ratings: [5, 4, 4, 5], createdAt: ISODate("2025-01-20T00:00:00.000Z") } ];

db.orders.aggregate([ { $lookup: { from: "products", localField: "products.productId", foreignField: "_id", pipeline: [ { $project: { name: 1, price: 1, brand: 1 } } ], as: "productDetails" } }, { $limit: 2 } ]);

[ { _id: 101, name: "iPhone 15", ratings: 5 }, { _id: 101, name: "iPhone 15", ratings: 4 }, { _id: 101, name: "iPhone 15", ratings: 5 }, { _id: 101, name: "iPhone 15", ratings: 3 } ];

db.orders.aggregate([ { $match: { // last 24 hours orderDate: { $gt: new Date(Date.now() - 24 * 60 * 60 * 1000) } } }, { $project: { products: 1 } }, { $unwind: "$products" }, { $lookup: { from: "products", localField: "products.productId", foreignField: "_id", as: "productDetails" } }, { $unwind: "$productDetails" }, { $group: { _id: "$productDetails.category", totalRevenue: { $sum: { $multiply: ["$products.quantity", "$productDetails.price"] } } } } ]);

Stage

Use

$match

Filter documents

$group

Group documents and calculate values

$project

Select, remove, rename, or transform fields

$sort

Sort documents

$limit

Limit number of documents

$skip

Skip documents

$unwind

Split array into multiple documents

$lookup

Join another collection

$addFields

Add new fields

$set

Same as $addFields

$unset

Remove fields

$count

Count total documents

$sample

Get random documents

$replaceRoot

Replace entire document

$replaceWith

Same as $replaceRoot

$sortByCount

Group, count, and sort documents

$unionWith

Combine multiple collections

$out

Save aggregation result into collection

$geoNear

Find nearby locations