Aggregation Framework

Aggregation is MongoDB’s pipeline system for transforming, joining, grouping, and analyzing documents.

Think of it as:

flowchart LR
    Docs[(Collection
    Raw Documents)] --> S1["Stage 1: $match
    Filter documents"]
    S1 --> S2["Stage 2: $group
    Group & aggregate"]
    S2 --> S3["Stage 3: $sort
    Sort results"]
    S3 --> Result["📊 Final Result
    Transformed data"]

    style Docs fill:#3b82f6,color:#fff
    style S1 fill:#7c3aed,color:#fff
    style S2 fill:#059669,color:#fff
    style S3 fill:#f59e0b,color:#fff
    style Result fill:#10b981,color:#fff

Each stage receives documents from the previous stage and sends transformed documents to the next stage.

documents -> stage -> stage -> stage -> result

Sections

Basic Syntax

db.collection.aggregate([
  { $match: { status: "completed" } },
  { $group: { _id: "$userId", total: { $sum: "$totalAmount" } } },
  { $sort: { total: -1 } }
])

Most Common Stages

Stage	Purpose
`$match`	Filter documents
`$project`	Include, exclude, rename, or compute fields
`$group`	Group documents and calculate totals
`$sort`	Sort result documents
`$limit`	Keep only a fixed number of documents
`$skip`	Skip documents, often for pagination
`$lookup`	Join with another collection
`$unwind`	Flatten array values
`$addFields`	Add computed fields