Mastering the MongoDB Aggregation Framework: Basics to Advanced

Mastering the MongoDB Aggregation Framework: Basics to Advanced

The MongoDB Aggregation Framework is a powerful tool for performing complex data transformations and computations. This guide covers the basics and advanced features of the Aggregation Pipeline.

What is Aggregation?

• Aggregation is a process of transforming data from a collection into aggregated results. It involves processing data records and returning computed results.

Aggregation Pipeline:

• A framework for data processing in MongoDB. It processes data through a series of stages that transform the data into the desired format.

Aggregation Stages

$match:

• Purpose: Filters documents based on specified criteria.

> db.orders.aggregate([{ $match: { status: "shipped" } }])

$group:

• Purpose: Groups documents by a specified field and performs aggregations (e.g., sum, average).

> db.orders.aggregate([
    { $group: { _id: "$category", totalAmount: { $sum: "$amount" } } }
  ])

$sort:

• Purpose: Sorts documents by specified fields.

> db.orders.aggregate([{ $sort: { totalAmount: -1 } }])

$project:

• Purpose: Reshapes documents by including or excluding fields.

> db.orders.aggregate([{ $project: { item: 1, totalAmount: 1 } }])

$limit:

• Purpose: Limits the number of documents passed to the next stage.

> db.orders.aggregate([{ $limit: 5 }])

$skip:

• Purpose: Skips a specified number of documents.

> db.orders.aggregate([{ $skip: 5 }])

Complex Aggregations:

$lookup:

• Purpose: Performs a left outer join to another collection.

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

$unwind:

• Purpose: Deconstructs an array field from the input documents.

> db.orders.aggregate([{ $unwind: "$items" }])

$arrayElemAt:

• Purpose: Returns the element at a specified array index.

> db.orders.aggregate([
    { $project: { firstItem: { $arrayElemAt: ["$items", 0] } } }
  ])

MongoDB Aggregation Framework, MongoDB Aggregation Pipeline, MongoDB $match, MongoDB $group, MongoDB $sort, MongoDB $project, MongoDB $lookup, MongoDB $unwind.