Add new field to every document in a MongoDB collection?

To add new field to every document in a MongoDB collection, you can use $set operator. The syntax is as follows ?

Syntax

db.yourCollectionName.update({}, { $set: {"yourFieldName": yourValue} }, false, true);

Sample Data

To understand the above syntax, let us create a collection with some documents. The query to create a collection with documents is as follows ?

db.addNewFieldToEveryDocument.insertMany([
    {"StudentName": "John", "StudentAddress": "US"},
    {"StudentName": "David", "StudentAddress": "UK"},
    {"StudentName": "Carol", "StudentAddress": "UK"},
    {"StudentName": "Bob", "StudentAddress": "US"}
]);

{
    "acknowledged" : true,
    "insertedIds" : [
        ObjectId("5c6efc0b6fd07954a48906ae"),
        ObjectId("5c6efc0b6fd07954a48906af"),
        ObjectId("5c6efc0b6fd07954a48906b0"),
        ObjectId("5c6efc0b6fd07954a48906b1")
    ]
}

Display all documents from a collection with the help of find() method. The query is as follows ?

db.addNewFieldToEveryDocument.find().pretty();

The following is the output ?

{
    "_id" : ObjectId("5c6efc0b6fd07954a48906ae"),
    "StudentName" : "John",
    "StudentAddress" : "US"
}
{
    "_id" : ObjectId("5c6efc0b6fd07954a48906af"),
    "StudentName" : "David",
    "StudentAddress" : "UK"
}
{
    "_id" : ObjectId("5c6efc0b6fd07954a48906b0"),
    "StudentName" : "Carol",
    "StudentAddress" : "UK"
}
{
    "_id" : ObjectId("5c6efc0b6fd07954a48906b1"),
    "StudentName" : "Bob",
    "StudentAddress" : "US"
}

Example

The following is the query to add a new field to every document ?

db.addNewFieldToEveryDocument.update({}, { $set: {"StudentAge": 24} }, false, true);

WriteResult({ "nMatched" : 4, "nUpserted" : 0, "nModified" : 4 })

Above, we have added a new field "StudentAge":24 in every document. Let us check the field "StudentAge":24 is successfully added to every document or not. The query is as follows ?

db.addNewFieldToEveryDocument.find().pretty();

The following is the output ?

{
    "_id" : ObjectId("5c6efc0b6fd07954a48906ae"),
    "StudentName" : "John",
    "StudentAddress" : "US",
    "StudentAge" : 24
}
{
    "_id" : ObjectId("5c6efc0b6fd07954a48906af"),
    "StudentName" : "David",
    "StudentAddress" : "UK",
    "StudentAge" : 24
}
{
    "_id" : ObjectId("5c6efc0b6fd07954a48906b0"),
    "StudentName" : "Carol",
    "StudentAddress" : "UK",
    "StudentAge" : 24
}
{
    "_id" : ObjectId("5c6efc0b6fd07954a48906b1"),
    "StudentName" : "Bob",
    "StudentAddress" : "US",
    "StudentAge" : 24
}

Conclusion

Use $set operator with an empty query filter {} to add a new field to every document in a MongoDB collection. The multi: true flag ensures all matching documents are updated.