$set (Set Fields)
Last updated: 2026-10-09
$set (Set Fields)
$set is an alias of $addFields: it adds or overwrites fields on each document while keeping everything else. Use it mid-pipeline to attach computed values, constants, or expression results. Introduced in MongoDB 3.4, it is fully interchangeable with $addFields; note it shares a name with the $set update operator.
| Category | Core Stages |
|---|---|
| MongoDB Version | 1.0+ |
| Example Language | mongosh (Shell) |
📝 Syntax
{ $set: { <field>: <expression>, ... } }⚙️ Parameters
| field | Field names mapped to aggregation expressions or constant values. |
|---|
Returns：Document stream with fields set
▶ Example
Run the commands below in your local mongosh (requires a running mongod):
// $set: add a constant status, convert amount type
db.orders.aggregate([
{ $set: { status: 'PENDING', amount: { $toDouble: '$amount' } } }
])Output：
// Original fields kept; status is a constant, amount converted to double📌 Notes
- $set is fully equivalent to $addFields
- _id and all unmentioned fields are untouched
- You cannot reference fields set in the same stage
🏷️ Related Tags
💡 Related Cases
More hands-on cases coming soon.
📚 Related Tutorials
❓ FAQ
QShould I use $set or $addFields?
AEither works; $addFields is more common, while $set can be confused with the update operator.
QCan $set set embedded fields?
AYes, with dotted paths, e.g. { $set: { 'addr.city': 'SH' } }.
QDoes $set delete fields?
ANo, it only adds or overwrites; use $unset to remove fields.
