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

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 } } );
});

Answer 2

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update/creation of a field based on another field:
// { "_id" : ObjectId("5e84c..."), "field1" : 12, "field2" : "world" }
db.collection.update(
{ "_id" : ObjectId("5e84c...") },
[{ $set: { field1: { $strLenCP: "$field2" } } }]
)
// { "_id" : ObjectId("5e84c..."), "field1" : 5, "field2" : "world" }
The first part {} is the match query, filtering which documents to update.
The second part [{ $set: { field1: { $strLenCP: "$field2" } } }] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline). $set is a new aggregation operator and an alias for $addFields. Any aggregation operator can be used within the $set stage; in our case $strLenCP which provides the length of field2.

Answer 3

As far I know the easiest way is the read and write aproach:
//At first, get/prepare your new value:
var d= db.yourColl.fetchOne({....});
d.field1== d.field2.length;
// then update with your new value
db.yourColl.save(d);

Answer 4

Your are using exists in the wrong way.
Syntax: { field: { $exists: <boolean> } }
You use of $where is also incorrect
Use the $where operator to pass either a string containing a JavaScript expression or a full JavaScript function to the query system
db.myCollection.find( { $where: "this.credits == this.debits" } );
db.myCollection.find( { $where: "obj.credits == obj.debits" } );
db.myCollection.find( { $where: function() { return (this.credits == this.debits) } } );
db.myCollection.find( { $where: function() { return obj.credits == obj.debits; } } );
I think you should use Map-Reduce for what you are trying to do.