What is the method for incorporating a variable in MapReduce?

I have the following MapReduce script that I'm working with:

splitAndGroupServices = function(groupMembers) {
    var mapFunction = function() {
       for(var idx in this.services) {
         var service = this.services[idx];
         if(service.member_id in groupMembers)
             emit(service.member_id, service);
       }
     }

     var reduceFunction = ...; 
     var finalizeFunction = ...;
     db.items.mapReduce(mapFunction, reduceFunction, {out: {inline:1}, finalize: finalizeFunction});
}

However, when I execute:

db.loadServerScripts();
splitAndGroupServices({b1: 0, b2: 1});

I encounter the error message:

"errmsg" : "exception: ReferenceError: groupMembers is not defined near 'ber_id in members) {             emit(ser'  (line 4)",

I am struggling to figure out how to pass a variable from an outer function into an inner function. While this is feasible in JavaScript, MongoDB seems to be resistant to it. Any suggestions on how to work around this limitation?

Answer №1

In the realm of the mapReduce command, there exists a feature known as "scope" that is shared throughout the mapper, reducer, and finalize stages:

db.items.mapReduce(
    mapFn,
    reduceFn, 
   { 
       "scope": { "members": members },
       "out": { "inline": 1 },
       "finalize": finalizeFn
   }
)

This element acts as a sort of global variable within the process, allowing any changes made to persist into subsequent stages or be accessible in those stages. It functions as a standard method for passing variables to be utilized within any of the stage functions.

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