Example custom operators for Mingo

gistfile1.js

/* 
 $pluck is a pipeline operator which returns the result of resolving `expr` on each item in the collection.
 This allows you to 'boil down' a collection to just the values you're interested in.

 Example:
 Mingo.aggregate(grades_complex, [{$unwind: "$scores"}, {$pluck: "scores.score"}]);
 returns an array of all scores in the 'grades_complex' dataset: [57.92947112575566, 21.24542588206755, 68.1956781058743, 67.95019716560351, 18.81037253352722, 39.17749400402234, ...]
*/
Mingo.addOperators(Mingo.OP_PIPELINE, function(m) {
    return {
        '$pluck': function(collection, expr) {
            return collection.map(function(item) {
                return m.resolve(item, expr);
            });            
        }
    };
});

/*
 $stddev is an operator which returns the standard deviation of all the values in a group.
 
 Example:
 Mingo.aggregate(grades_complex, [{$unwind: "$scores"}, {$group: {$stddev: "$scores.score"}}]);
 returns: [{$stddev: 28.57362029450366}]
*/
Mingo.addOperators(Mingo.OP_GROUP, function(m) {
    return {
        '$stddev': function(collection, expr) {
            var avg = this.$avg(collection, expr);
            var diffs = _.map(collection, function(item) {var v = m.computeValue(item, expr) - avg; return v*v});
            var variance = _.reduce(diffs, function (memo, val) {return memo + val}, 0) / diffs.length;
            return Math.sqrt(variance);        
        }
    };
});