How to copy attributes in MongoDB?

To copy the value of one attribute to another in MongoDB, use $set with forEach() to iterate through documents and update each one by copying the source field value to the target field.

Syntax

db.collection.find({}).forEach(function(doc) {
    db.collection.update(
        { _id: doc._id }, 
        { $set: { targetField: doc.sourceField } }
    );
});

Sample Data

db.demo55.insertMany([
    { "ShippingDate": "", "date": new ISODate("2019-01-21") },
    { "ShippingDate": "", "date": new ISODate("2020-05-12") }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e2716dfcfb11e5c34d89915"),
        ObjectId("5e2716ebcfb11e5c34d89916")
    ]
}

Display all documents from the collection ?

db.demo55.find();

{ "_id": ObjectId("5e2716dfcfb11e5c34d89915"), "ShippingDate": "", "date": ISODate("2019-01-21T00:00:00Z") }
{ "_id": ObjectId("5e2716ebcfb11e5c34d89916"), "ShippingDate": "", "date": ISODate("2020-05-12T00:00:00Z") }

Example: Copy date to ShippingDate

Copy the date field value to the ShippingDate field for all documents ?

db.demo55.find({}).forEach(function(doc) {
    db.demo55.update(
        { _id: doc._id }, 
        { $set: { ShippingDate: doc.date } }
    );
});

Verify Result

db.demo55.find();

{ "_id": ObjectId("5e2716dfcfb11e5c34d89915"), "ShippingDate": ISODate("2019-01-21T00:00:00Z"), "date": ISODate("2019-01-21T00:00:00Z") }
{ "_id": ObjectId("5e2716ebcfb11e5c34d89916"), "ShippingDate": ISODate("2020-05-12T00:00:00Z"), "date": ISODate("2020-05-12T00:00:00Z") }

Key Points

• forEach() iterates through each document in the collection.
• $set updates the target field with the source field value.
• Use doc._id to identify each document for individual updates.

Conclusion

The forEach() method combined with $set allows you to copy attribute values between fields across all documents in a collection. This approach ensures each document is updated individually with its own field values.