MongoDB $setField Operator

What Is $setField?

$setField is a MongoDB aggregation expression that returns a copy of an object with one field set to a value. The field name can be fixed or dynamic (from another field). Pair it with $getField to read that value back.

$setField = object + field = value  (returns new object)

{ $setField: {
    field: <name string>,
    input: <object>,
    value: <expression>
  }
}

existing field → replaced
missing field  → added
value $$REMOVE → field dropped

$input is copied — original expression not mutated

How to Use $setField in MongoDB

Name the field, pass the object, and supply the new value.

1. Choose an expression stage ($addFields or $project).
2. Set input to the object (for example "$profile").
3. Set field to a string name (literal or "$key").
4. Set value to the new contents (or "$$REMOVE" to drop the field).

Pseudocode

obj  = evaluate(input)
name = evaluate(field)     // must be a string
val  = evaluate(value)

result = shallow_copy(obj)
if val is $$REMOVE:
    delete result[name]
else:
    result[name] = val

emit { ..., field: result }

Cheat Sheet

Keep these facts handy while updating objects by field name.

Live Preview

Start from { city: "Chennai" } and apply $setField-style updates.

input = { city: "Chennai" }
field = "age"
value = 14

→ { city: "Chennai", age: 14 }

Worked Walkthrough — Add Age to Profile

Asha’s profile only has city. We add age: 14 with $setField and keep the city.

input = { city: "Chennai" }
field = "age"
value = 14

copy object
set age → 14

result = { city: "Chennai", age: 14 }

(original profile expression is not changed;
 we assign the result back with $addFields)

MongoDB Examples

We use one student with a nested profile. First insert the sample, then add age, then set a field whose name comes from another field. Run these in mongosh after inserting the sample data once.

db.students.insertOne({
  _id: 1,
  name: "Asha",
  key: "score",
  profile: { city: "Chennai" }
});

db.students.find()

{
  _id: 1,
  name: "Asha",
  key: "score",
  profile: { city: "Chennai" }
}

profile: { city: "Chennai" }
              +
         age: 14
              ↓
profile: { city: "Chennai", age: 14 }

db.students.aggregate([
  {
    $addFields: {
      profile: {
        $setField: {
          field: "age",
          input: "$profile",
          value: 14
        }
      }
    }
  }
]);

{
  _id: 1,
  name: "Asha",
  key: "score",
  profile: { city: "Chennai", age: 14 }
}

db.students.aggregate([
  {
    $addFields: {
      profile: {
        $setField: {
          field: "$key",
          input: "$profile",
          value: 95
        }
      }
    }
  }
]);

{
  _id: 1,
  name: "Asha",
  key: "score",
  profile: { city: "Chennai", score: 95 }
}

Performance Notes

Prefer plain $addFields when the field name is fixed. Reach for $setField when the name is dynamic or hard to express with dots.

Key Takeaways