MongoDB Aggregation Pipeline Explained Step by Step

A MongoDB aggregation pipeline lets you process data step by step. Instead of writing one huge query, you build a sequence of small steps, and each one changes the result a little more. When you first encounter an aggregation query, it often looks something like this: db . enrollments . aggregate ([ { $match : { status : " active " } }, { $group : { id : " $courseId " , enrollmentsCount : { $sum : 1 } } }, { $lookup : { from : " courses " , localField : " id " , foreignField : " id " , as : " course " } }, { $unwind : " $course " }, { $project : { courseTitle : " $course.title " , enrollmentsCount : 1 , id : 0 } }, { $sort : { enrollmentsCount : - 1 } }, { $limit : 5 } ]) If you are new to aggregation pipelines, a query like this can feel hard to follow at first. When the same logic is built visually, it becomes much easier to understand because you can see how the result changes step by step. A simple way to think about it is like this: documents → filter → group → join→ shape → sort