Mongo - How to update a property of a document using a nested value of the same document

Question

I want to update a document using a nested value from that same document. So if I have an document that looks like this:

{
  _id: ObjectId("60b002d490b01b63eee5d4b7"),
  title: 'foo',
  meta: {
    createdTime: ISODate("2011-07-06T17:53:41.439Z"),
    updatedTime: ISODate("2021-07-06T17:53:41.432Z")
  }
}

I want to run a database operation that will copy the meta.updatedTime to another field (that we can call newProperty) on the document so that the end result would be this:

{
  _id: ObjectId("60b002d490b01b63eee5d4b7"),
  title: 'foo',
  meta: {
    createdTime: ISODate("2011-07-06T17:53:41.439Z"),
    updatedTime: ISODate("2021-07-06T17:53:41.432Z")
  },
  newProperty: ISODate("2021-07-06T17:53:41.432Z")
}

I know that I can do a find to get the document and then a separate update, but I was hoping to be able to do it all in one operation. This way if I'm running this operation on 50,000 or 100,000 records, I get the sense that a simple db operation would be faster than looping through the records and performing an operation on each one using javascript. But that might be negligible? Any help is appreciated.

Answer 1

If you get error Unrecognized pipeline stage name: '$addField' when using $addField, give the below statement a shot.

db.collection.updateMany(
    {},
    [
       { $set: { newProperty: "$meta.createdTime" } }
    ]
)

Answer 2

Try this:

db.collection.updateMany(
    {}, 
    [ 
       { $addFields: { newProperty: "$meta.updatedTime" } } 
    ]
)