Updating a document based on another field

[Ravi_Tatikonda1 (RkCool)]

How can I use update() to update all documents by adding a new field based on value of an existing field in same document

I tried to use $ in value of $set but it didnt work

ex:

db.dummycoll.insertOne({f1:1,f2:2})

I want to update all documents to add new field f3 which should be the value of f2

db.dummycoll.update({},
{$set:{f3:’$f2’}})

above didnt work , it is setting string $f2

[steevej (Steeve Juneau)]

See the answer from this thread:

How can we assign one field value to another?

Hi @Dhara_Pathak, Starting from MongoDB 4.2 you can perform Updates with an Aggregation Pipeline. An aggregation pipeline enables more expressive updates including calculated fields and references to other field values in the same document. You haven’t provided an example document, but based on your description I assume something like the following would be representative: db.mydata.insert({ invoices: { services: { price: NumberDecimal("4.99") } } }) Updat…