Update MongoDB variable value with variable itself?

To update a MongoDB field value using its current value as part of the new value, use the $set operator with string concatenation or the $concat aggregation operator within an update pipeline.

Syntax

// Using aggregation pipeline (recommended)
db.collection.updateMany(
    {},
    [{ $set: { fieldName: { $concat: ["$fieldName", " additional text"] } } }]
);

// Using $set with literal string (replaces with literal text)
db.collection.update(
    {},
    { $set: { fieldName: "fieldName additional text" } }
);

Sample Data

db.demo256.insertMany([
    { "Name": "Chris" },
    { "Name": "Bob" },
    { "Name": "David" }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e47a3e91627c0c63e7dba8b"),
        ObjectId("5e47a3ea1627c0c63e7dba8c"),
        ObjectId("5e47a3eb1627c0c63e7dba8d")
    ]
}

Method 1: Using Aggregation Pipeline (Recommended)

Update all documents to append " is a student" to existing Name values ?

db.demo256.updateMany(
    {},
    [{ $set: { "Name": { $concat: ["$Name", " is a student"] } } }]
);

{ "acknowledged": true, "matchedCount": 3, "modifiedCount": 3 }

Method 2: Using $set with Literal String

This approach replaces the field with a literal string value ?

db.demo256.update(
    {},
    { $set: { "Name": "Name is a student" } }
);

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Results

db.demo256.find();

{ "_id": ObjectId("5e47a3e91627c0c63e7dba8b"), "Name": "Chris is a student" }
{ "_id": ObjectId("5e47a3ea1627c0c63e7dba8c"), "Name": "Bob is a student" }
{ "_id": ObjectId("5e47a3eb1627c0c63e7dba8d"), "Name": "David is a student" }

Key Differences

• Aggregation pipeline with $concat uses the actual field value ($Name)
• Literal $set replaces with the string "Name" (not the field value)
• updateMany() affects all matching documents, update() affects only the first

Conclusion

Use aggregation pipelines with $concat to update fields using their current values. The $set operator with literal strings only replaces with fixed text, not the actual field content.