Collection.add accepts negative indexes for the at option

backbone.js

@@ -700,6 +700,7 @@
       models = singular ? (models ? [models] : []) : models.slice();
       var id, model, attrs, existing, sort;
       var at = options.at;
+      if (at < 0) at += this.length + 1;
       var sortable = this.comparator && (at == null) && options.sort !== false;
       var sortAttr = _.isString(this.comparator) ? this.comparator : null;
       var toAdd = [], toRemove = [], modelMap = {};

test/collection.js

@@ -1482,4 +1482,11 @@
     collection.set([{id: 1}, {id: 2}, {id: 3}]);
   });
 
+  test("add supports negative indexes", 1, function() {
+    var collection = new Backbone.Collection([{id: 1}]);
+    collection.add([{id: 2}, {id: 3}], {at: -1});
+    collection.add([{id: 2.5}], {at: -2});
+    equal(collection.pluck('id').join(','), "1,2,2.5,3");
+  });
+
 })();