Convert ObjectID to String in mongo Aggregation

Question

I\'m in this scenario right now: I have a collection X:

{
  _id:ObjectId(\'56edbb4d5f084a51131dd4c6\'),
  userRef:ObjectId(\'56edbb4d5f084a51131dd4c6\'),
  seria         


        

Answer 1

I couldn't find a way to do what I wanted, so instead, I created a MapReduce function that, in the end, generated the keys the way I wanted to (concatenating other keys).

At the end, it looked something like this:

db.collection('myCollection').mapReduce(
    function() {
        emit(
            this.userRef.str + '-' + this.serialNumber , {
                count: 1,
                whateverValue1:this.value1,
                whateverValue2:this.value2,
                ...
            }
        )
    },
    function(key, values) {
       var reduce = {}
       .... my reduce function....
        return reduce
    }, {
        query: {
            ...filters_here....
        },
        out: 'name_of_output_collection'
    }
);