$project, $sort, $limit

Reshape output fields with $project, order results with $sort, and cap them with $limit and $skip.

$project: Reshaping Documents

$project controls which fields appear in the output, exactly like find()'s projection argument, but with far more power — it can also compute entirely new fields from expressions.

db.orders.aggregate([
  { $project: { customerId: 1, total: 1, _id: 0 } },
]);

Computing New Fields with $project

db.orders.aggregate([
  {
    $project: {
      customerId: 1,
      total: 1,
      totalWithTax: { $multiply: ["$total", 1.08] },
      isLargeOrder: { $gt: ["$total", 100] },
    },
  },
]);

A Rich Expression Language

Beyond $multiply and $gt shown here, MongoDB's aggregation expressions include string manipulation ($concat, $toUpper), date extraction ($year, $month), conditionals ($cond), and dozens more — covering most computation needs directly in the pipeline.

$sort

$sort orders documents flowing through the pipeline, using the same 1/-1 convention as find().sort().

db.orders.aggregate([
  { $group: { _id: "$customerId", totalSpent: { $sum: "$total" } } },
  { $sort: { totalSpent: -1 } }, // highest spenders first
]);

$limit and $skip

$limit caps the number of documents passed to the next stage; $skip discards a number of leading documents — together enabling pagination within a pipeline.

db.orders.aggregate([
  { $group: { _id: "$customerId", totalSpent: { $sum: "$total" } } },
  { $sort: { totalSpent: -1 } },
  { $skip: 10 },
  { $limit: 10 }, // "page 2" of top spenders
]);

FAQs

Does stage order matter for $sort and $limit?

Yes — placing $limit before $sort would cap the documents before they're ordered, likely giving the wrong result; always $sort before $limit when you want the top N by some criteria.

Can $project remove fields that were computed earlier in the pipeline, like from $group?

Yes — $project (or a later stage) can reshape or drop any field present at that point in the pipeline, including ones computed by earlier stages.

Summary

$project reshapes and computes fields, while $sort/$limit/$skip control ordering and pagination of pipeline output. Next, you'll learn $lookup, MongoDB's equivalent of a relational join.

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