Why should you care?

APIs and reports rarely need every field. Computing fullName or lineTotal in the pipeline keeps app code thinner.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.users.insertOne({ first: "Asha", last: "Nair", passwordHash: "x", city: "Kochi" })
db.users.aggregate([
  { $project: {
      _id: 0,
      name: { $concat: ["$first", " ", "$last"] },
      city: 1
  }}
])

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• passwordHash is dropped by omission.
• name is computed with $concat.
• city is kept.
• _id: 0 hides the id.

Practice next

1. Run the pipeline.
2. Add a field isCoastal: { $eq: ["$city", "Kochi"] }.
3. Contrast with find projection — aggregation expressions are richer.
4. Compute priceWithTax: { $multiply: ["$price", 1.18] } on products.
5. Rename total to amount with total: 0, amount: "$total" patterns carefully.

Remember

$project shapes fields. You can compute new values. Drop secrets early.

Export users for email tool

Marketing export projects email and name only.

Outcome: CSV never contains password hashes.