Write an aggregation pipeline that returns the top three products by revenue.
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of MongoDB conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
Pipelines pass documents through stages, transforming them step by step.
db.orders.aggregate([
{ $match: { status: "paid" } },
{ $unwind: "$items" },
{ $group: {
_id: "$items.productId",
revenue: { $sum: { $multiply: ["$items.price", "$items.qty"] } }
}},
{ $sort: { revenue: -1 } },
{ $limit: 3 },
{ $lookup: {
from: "products",
localField: "_id",
foreignField: "_id",
as: "product"
}},
{ $project: { _id: 0, name: { $first: "$product.name" }, revenue: 1 } }
]);
$match first so later stages process fewer documents. $unwind flattens the items array. $group aggregates by product, $sort and $limit select the top three, and $lookup enriches with product names. Put the most selective stages early and project away fields you no longer need.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.