mongodb中的group功能

Mysql:

select a,b,sum(c) csum from coll where active=1 group by a,b

MongoDB:

db.coll.group(
           {key: { a:true, b:true },
            cond: { active:1 },
            reduce: function(obj,prev) { prev.csum += obj.c; },
            initial: { csum: 0 }
            });