Aggregation Pipeline: $project and $unwind

1. Introduction

The MongoDB Aggregation Pipeline is a powerful framework that allows for transforming and processing data. Two key stages of this pipeline are $project and $unwind, which are essential for reshaping the data structure.

2. Understanding $project

The $project stage reshapes each document in the pipeline, allowing you to include, exclude, or add new fields. It is particularly useful for controlling the structure of the output documents.

2.1 Key Concepts

• Includes fields: Specify which fields to return.
• Excludes fields: Specify which fields to omit.
• Computed Fields: Create new fields based on existing ones.

2.2 Example

db.orders.aggregate([
    {
        $project: {
            item: 1,
            total: { $multiply: ["$price", "$quantity"] },
            _id: 0
        }
    }
]);
                

This example projects the item field and creates a new field total by multiplying price and quantity.

3. Understanding $unwind

The $unwind stage deconstructs an array field from the input documents to output a document for each element. This is useful when you have arrays and want to normalize them into individual documents.

3.1 Key Concepts

• Flattening Arrays: Each element in the array becomes a separate document.
• Preserving Context: Other fields in the document are preserved.

3.2 Example

db.students.aggregate([
    {
        $unwind: "$courses"
    }
]);
                

In this example, the courses array in each student document is unwound, creating a separate document for each course.

4. Best Practices

• Use $project early to reduce document size.
• Filter data as early as possible using $match.
• Avoid using $unwind on large arrays unless necessary.

5. FAQ