aggregate:
  "function(obj, prev) {
     prev.count++;
   }"

group:
  "function(obj, prev) {
     prev.group.push(obj);
   }"

max:
  "function(obj, prev) {
     if (prev.max == 'start') {
       prev.max = obj.[field];
     }
     if (prev.max < obj.[field]) {
       prev.max = obj.[field];
     }
   }"

min:
  "function(obj, prev) {
     if (prev.min == 'start') {
       prev.min = obj.[field];
     }
     if (prev.min > obj.[field]) {
       prev.min = obj.[field];
     }
   }"

sum:
  "function(obj, prev) {
     if (prev.sum == 'start') {
       prev.sum = 0;
     }
     prev.sum += obj.[field];
  }"