mongodb convert array elements from int to string - mongodb

Question

I have a mongodb full of records with mixed types and need to sanitize data to make some sense in back end application.
To change type(NumberLong to String) of a normal field is easy, just cast it to string in Javascript like:
db.foo.find({ field: { $type: 18 } }).forEach(function (x) {
x.field = new String(x.field); // convert field to string
db.foo.save(x);
});
But how do I change array elements from NumberLong to String?
For example I have field:
"elements" : { "top" : {"random" : [ NumberLong(12) , NumberLong(20), NumberLong(13)] } }
and I need all the elements of elements.top.random as strings.
Do I need to do "foreach" element of the array or is there any better way?
Any Javascript guru can help me out?

Answer 1

Before posting I just could not find an answer, and now I did find something similar and adopted...
so here it is:
db.foo.find( {"elements.top.random": {$type:18}} ).forEach( function (x) {
var arr = [];
x.elements.top.random.forEach( function (e) { arr.push("" + e); } );
x.elements.top.random = arr;
db.foo.save(x);
});

Answer 2

We came across this issue today with a more recent version of MongoDB (v4.0.0). Since the save() API is deprecated, we had to use updateOne(), resulting in the following code:
db.foo.find( {"elements.top.random": {$type:18}} ).forEach(function (x) {
var stringValues = [];
x.elements.top.random.forEach(function(e) {
stringValues.push("" + e);
});
db.foo.updateOne(
{ _id: x._id },
{ $set: { "elements.top.random" : stringValues } }
);
});

Answer 3

With MongoDB v4.2+, you can do an update with aggregation pipeline. Use $reduce and $toString to do the string conversion and string concat.
db.collection.update({
"elements.top.random": {
$type: 18
}
},
[
{
$set: {
"elements.top.random": {
"$reduce": {
"input": "$elements.top.random",
"initialValue": "",
"in": {
"$concat": [
"$$value",
{
$toString: "$$this"
}
]
}
}
}
}
}
],
{
multi: true
})
Here is the Mongo playground for your reference.
With a slightly older version of MongoDB (v4.0+), you can still have $toString available. You can use the old "aggregate-then-update" approach.
db.collection.aggregate([
{
$match: {
"elements.top.random": {
$type: 18
}
}
},
{
$set: {
"elements.top.random": {
"$reduce": {
"input": "$elements.top.random",
"initialValue": "",
"in": {
"$concat": [
"$$value",
{
$toString: "$$this"
}
]
}
}
}
}
}
]).forEach(agg => {
db.collection.update(
{ _id: agg._id },
{ $set: { "elements.top.random" : agg.elements.top.random } }
)
})