Javascript Shell Example

// store:
db.system.js.save( 
{
    _id : "sumBalance",
    value: 
        function (f) { 
            var s = 'db.customers.aggregate(
                        {$group: { _id : "$' + f + '",
                                  bal : {$sum:"$acct_balance"}}},
                        {$sort : {_id:1}});'; 
            return db.eval(s); 
        }
});
// execute:
db.eval("sumBalance('status')");