Convert a string to a number in MongoDB projection

Question

I have a collection of documents that have a value that is known to be a number, but is stored as a string. It is out of my control to change the type of the field, but I want to use that field in an aggregation (say, to average it).

It seems that I should be using a projection prior to grouping, and in that projection convert the field as needed. I can't seem to get the syntax just right - everything I try either gives me NaN, or the new field is simply missing from the next step in the aggregation.

$project: {
    value: '$value',
    valueasnumber: ????
}

Given the very simple example above, where the contents of $value in all documents are string type, but will parse to a number, what do I do to make valueasnumber a new (non-existing) field that is of type double with the parsed version of $value in it?

I've tried things like the examples below (and about a dozen similar things):

{ $add: new Number('$value').valueOf() }
new Number('$value').valueOf()

Am I barking up the wrong tree entirely? Any help would be greatly appreciated!

(To be 100% clear, below is how I would like to use the new field).

$group {
    score: {
        $avg: '$valueasnumber'
    }
}

Answer 1

One of the way which I can think of is to use a mongo shell javascript to modify the document by adding new number field, valuesasnumber (number conversion of existing string 'value' field) in the existing document or in the new doc. Then using this numeric field for further calculations.

db.numbertest.find().forEach(function(doc) {
    doc.valueasnumber = new NumberInt(doc.value);
    db.numbertest.save(doc);
});

Using the valueasnumber field for numeric calculation

db.numbertest.aggregate([{$group : 
   {_id : null, 
    "score" : {$avg : "$valueasnumber"}
   }
}]);