mongodb/jstests/group3.js

t = db.group3;
t.drop();

t.save({a: 1});
t.save({a: 2});
t.save({a: 3});
t.save({a: 4});


cmd = { initial: {count: 0, sum: 0},
        reduce: function(obj, prev) {
            prev.count++;
            prev.sum += obj.a;
        },
        finalize: function(obj) {
            if (obj.count){
                obj.avg = obj.sum / obj.count;
            }else{
                obj.avg = 0;
            }
        },
      };

result1 = t.group(cmd);

assert.eq(1, result1.length, "test1");
assert.eq(10, result1[0].sum, "test1");
assert.eq(4, result1[0].count, "test1");
assert.eq(2.5, result1[0].avg, "test1");


cmd['finalize'] = function(obj) {
    if (obj.count){
        return obj.sum / obj.count;
    }else{
        return 0;
    }
};

result2 = t.group(cmd);

assert.eq(1, result2.length, "test2");
assert.eq(2.5, result2[0], "test2");