MongoDB: Problems using $concat to update the value of a field - mongodb

Question

I'm trying to update the value of a field in a MongoDB collection by concatenating it with a literal string. Besides this, the field is an integer, and I want to add a "0" in front, so it will became a string.
I've read that I can't use the old value of the field in a single update instruction, so I'm using a forEach() method.
Here is the code:
db.col_1.find({"field_1": {$lt: 10000}}).forEach( function(i){
db.col_1.update({_id: i._id},
{$set: { "field_1": {$concat: ["0", i.field_1]}}}
)
});
The return result is :
The dollar ($) prefixed field '$concat' in 'field_1.$concat' is not valid for storage.
I'm sure I'm not writting the $concat command properly, is there any way to do this?

Answer 1

$concat is an aggregation pipeline, not an update operator/modifier.
It seems that what you're trying to do can be achieved by doing the following:
db.col_1
.find({ "field_1": { $lt: 10000 } })
.forEach( function(i) {
db.col_1.update(
{ _id: i._id },
{ $set: { "field_1": "0" + i.field_1 } }
)
});

Answer 2

To update the MongoDB field using the value of another field for MongoDB version 4.2 also introduced the $set pipeline stage operator which is an alias for $addFields. You can use $set here as it maps with what we are trying to achieve.
let query = {
"field_1": {
$lt: 10000
}
};
let changes = {
$set: {
"field_1": {
"$concat": ["0", "$field_1"]
}
}
};
let UpdatedStatus = await col_1.updateMany(query, [changes]).lean();
console.log(UpdatedStatus);