MongoDB update. Trying to set one field from a property of another

Question

What I'm trying to do is pretty straightforward, but I can't find out how to give one field the value of another.

I simply want to update one field with the character count of another.

db.collection.update({$exists:true},{$set : {field1 : field2.length}})

I've tried giving it dot notation

db.collection.update({$exits:true},{$set : {field1: "this.field2.length"}})

As well as using javascript syntax

db.collection.update({$exits:true},
                     {$set : {field1: {$where : "this.field2.length"}})

But just copied the string and got a "notOkforstorage" respectively. Any help?

Update: I only get the "notOkforStorage" when I query by ID:

db.collection.update({_id:ObjectID("38289842bbb")},
                     {$set : {field1: {$where :"this.field2.length"}}})

Answer 1

Try the following code:

db.collection.find(your_querry).forEach(function(doc) {
  doc.field1 = doc.field2.length;
  db.collection.save(doc);
});

You can use your_querry to select only part of the original collection do perform an update. If you want to process an entire collection, use your_querry = {}.

If you want all operations to be atomic, use update instead of save:

db.collection.find( your_querry, { field2: 1 } ).forEach(function(doc) {
  db.collection.update({ _id: doc._id },{ $set: { field1: doc.field2.length } } );
});