How to use a variable for mapReduce in MongoDB with NodeJS

I have a collection of events that look like

{
  _id: BSONID
  name: "event_name",
  values: {a: 10, b: 1000, c: 50}
}

I'm trying to use mapReduce them using

map = function() {
  return emit([this.name, this.values['a']], this.values['b']);
}

reduce = function(key, values) {
  // stuff
}

collection.mapReduce(map, reduce, { out: { inline: 1 } }, callback);

However, I would like to be able to dynamically change which values I map against. In essence, I'd like to have

var key = 'a';
var value = 'b';

map = function ()
{
  return emit([this.name, this.values[key]], this.values[value]);
}

The problem is that the execution context isn't passed to mongodb. Any solution that doesn't rely on using string for functions?