Adding the ability to _.groupBy(list, 'account_id')

test/collections.js

@@ -202,6 +202,12 @@ $(document).ready(function() {
     var parity = _.groupBy([1, 2, 3, 4, 5, 6], function(num){ return num % 2; });
     ok('0' in parity && '1' in parity, 'created a group for each value');
     equals(parity[0].join(', '), '2, 4, 6', 'put each even number in the right group');
+
+    var list = ["one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten"];
+    var grouped = _.groupBy(list, 'length');
+    equals(grouped['3'].join(' '), 'one two six ten');
+    equals(grouped['4'].join(' '), 'four five nine');
+    equals(grouped['5'].join(' '), 'three seven eight');
   });
 
   test('collections: sortedIndex', function() {

underscore.js

@@ -276,9 +276,11 @@
     }), 'value');
   };
 
-  // Groups the object's values by a criterion produced by an iterator
-  _.groupBy = function(obj, iterator) {
+  // Groups the object's values by a criterion. Pass either a string attribute
+  // to group by, or a function that returns the criterion.
+  _.groupBy = function(obj, val) {
     var result = {};
+    var iterator = _.isFunction(val) ? val : function(obj) { return obj[val]; };
     each(obj, function(value, index) {
       var key = iterator(value, index);
       (result[key] || (result[key] = [])).push(value);