Introduction to Aggregation

The Aggregation Framework is a powerful feature of MongoDB that allows for data transformation, filtering, and analysis. In this guide, we'll explore advanced techniques using the Aggregation Framework to perform complex operations.

1. `$group` Stage

The `$group` stage is used to group documents based on specified criteria. Here's an example of using the `$group` stage to calculate the average price of products:

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

2. `$lookup` Stage

The `$lookup` stage is used to perform a left outer join between two collections. It's handy for combining data from different sources. Here's an example of using the `$lookup` stage:

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

3. `$unwind` Stage

The `$unwind` stage is used to deconstruct arrays within documents, enabling further analysis. Here's an example of using the `$unwind` stage:

db.sales.aggregate([
    {
        $unwind: "$items"
    }
]);
    

4. `$facet` Stage

The `$facet` stage allows you to perform multiple aggregation pipelines and return their results as an array. It's useful for running several operations in parallel. Here's an example of using the `$facet` stage:

db.collection.aggregate([
    {
        $facet: {
            stage1: [/* Aggregation pipeline 1 */],
            stage2: [/* Aggregation pipeline 2 */],
        }
    }
]);
    

Conclusion

The Advanced Aggregation Framework in MongoDB provides a rich set of tools for data analysis and transformation. By mastering techniques like `$group`, `$lookup`, `$unwind`, and `$facet`, you can perform complex operations on your data. Experiment with these stages to unlock the full potential of MongoDB's Aggregation Framework.