Issue #209 ... unsetting a missing attribute should not fire a change event.

backbone.js

@@ -206,8 +206,9 @@
     },
 
     // Remove an attribute from the model, firing `"change"` unless you choose
-    // to silence it.
+    // to silence it. `unset` is a noop if the attribute doesn't exist.
     unset : function(attr, options) {
+      if (!(attr in this.attributes)) return this;
       options || (options = {});
       var value = this.attributes[attr];
 

test/model.js

@@ -152,6 +152,17 @@ $(document).ready(function() {
     equals(a.id, undefined, "Unsetting the id should remove the id property.");
   });
 
+  test("Model: multiple unsets", function() {
+    var i = 0;
+    var counter = function(){ i++; };
+    var model = new Backbone.Model({a: 1});
+    model.bind("change:a", counter);
+    model.set({a: 2});
+    model.unset('a');
+    model.unset('a');
+    equals(i, 2, 'Unset does not fire an event for missing attributes.');
+  });
+
   test("Model: using a non-default id attribute.", function() {
     var MongoModel = Backbone.Model.extend({idAttribute : '_id'});
     var model = new MongoModel({id: 'eye-dee', _id: 25, title: 'Model'});