MongoDB $set Stage

What Is $set?

$set is a MongoDB aggregation stage that adds new fields or overwrites existing ones using expressions. Other fields stay on the document. It is an alias for $addFields.

$set = add or overwrite fields, keep the rest

{ $set: { <field>: <expression>, ... } }

keeps all existing fields
same name → overwrites that field
alias for $addFields (same behavior)

vs $project → reshape / pick fields
not the update operator $set

How to Use $set in MongoDB

Name the fields, give each an expression, and place the stage where later steps need those values.

1. Start (or continue) an aggregation pipeline with aggregate([...]).
2. Add { $set: { fieldName: <expression> } }.
3. Use field paths like "$price" and operators like $multiply.
4. Use the new or updated fields in later stages.

Pseudocode

for each document:
    out = copy(document)          // keep existing fields
    for each (field, expression) in $set:
        out[field] = evaluate(expression)   // add or overwrite
    emit out

Cheat Sheet

Keep these facts handy while setting fields in a pipeline.

Live Preview

Watch $set add salePrice = price × 0.9 while keeping the original fields.

input:
  { name: "Mug", price: 20, status: "draft" }

$multiply → 20 * 0.9 = 18

output (+ keeps original fields):
  { name: "Mug", price: 20, status: "draft", salePrice: 18 }

Worked Walkthrough — Sale Price

One product has price: 20 and status: "draft". Goal: add salePrice without dropping other fields.

Document in:
  { name: "Mug", price: 20, status: "draft" }

$set:
  salePrice: { $multiply: ["$price", 0.9] }

20 * 0.9 = 18

Document out:
  {
    name: "Mug",
    price: 20,
    status: "draft",
    salePrice: 18      // new
  }

MongoDB Examples

We use a small products collection. First add a sale price, then overwrite status and add the sale price together. Run these in mongosh after inserting the sample data once.

db.products.insertMany([
  { _id: 1, name: "Mug", price: 20, status: "draft" },
  { _id: 2, name: "Notebook", price: 10, status: "draft" },
  { _id: 3, name: "Bag", price: 40, status: "draft" }
]);

db.products.find()

[
  { _id: 1, name: "Mug", price: 20, status: "draft" },
  { _id: 2, name: "Notebook", price: 10, status: "draft" },
  { _id: 3, name: "Bag", price: 40, status: "draft" }
]

db.products.aggregate([
  {
    $set: {
      salePrice: { $multiply: ["$price", 0.9] }
    }
  }
]);

{
  _id: 1,
  name: "Mug",
  price: 20,
  status: "draft",
  salePrice: 18
}
{
  _id: 2,
  name: "Notebook",
  price: 10,
  status: "draft",
  salePrice: 9
}
{
  _id: 3,
  name: "Bag",
  price: 40,
  status: "draft",
  salePrice: 36
}

db.products.aggregate([
  {
    $set: {
      status: "listed",
      salePrice: { $multiply: ["$price", 0.9] }
    }
  }
]);

{
  _id: 1,
  name: "Mug",
  price: 20,
  status: "listed",
  salePrice: 18
}

Performance Notes

Expression work is usually cheap compared with large sorts or joins. Filter early with $match when only some documents need the new fields.

Key Takeaways