Only sort on add when it make sense

backbone.js

@@ -619,7 +619,7 @@
     // Add a model, or list of models to the set. Pass **silent** to avoid
     // firing the `add` event for every new model.
     add: function(models, options) {
-      var i, args, length, model, existing;
+      var i, args, length, model, existing, sort;
       var at = options && options.at;
       models = _.isArray(models) ? models.slice() : [models];
 
@@ -639,6 +639,7 @@
         if (existing || this._byCid[model.cid]) {
           if (options && options.merge && existing) {
             existing.set(model, options);
+            sort = true;
           }
           models.splice(i, 1);
           continue;
@@ -651,14 +652,15 @@
         if (model.id != null) this._byId[model.id] = model;
       }
 
-      // Update `length` and splice in new models.
+      // See if sorting is needed, update `length` and splice in new models.
+      if (models.length) sort = true;
       this.length += models.length;
       args = [at != null ? at : this.models.length, 0];
       push.apply(args, models);
       splice.apply(this.models, args);
 
       // Sort the collection if appropriate.
-      if (this.comparator && at == null) this.sort({silent: true});
+      if (sort && this.comparator && at == null) this.sort({silent: true});
 
       if (options && options.silent) return this;
 

test/collection.js

@@ -715,4 +715,18 @@ $(document).ready(function() {
     this.ajaxSettings.success([model]);
   });
 
+  test("`sort` shouldn't always fire on `add`", 1, function() {
+    var c = new Backbone.Collection([{id: 1},{id: 2},{id: 3}], {
+      comparator: 'id'
+    });
+    c.sort = function() {
+      ok(1);
+      return Backbone.Collection.prototype.sort.apply(this, arguments);
+    }
+    c.add([]);
+    c.add({id: 1});
+    c.add([{id: 2},{id: 3}]);
+    c.add({id: 4});
+  });
+
 });