gistfile1.js

    handleTagChange: function(tags) {
      var tagCollection = this.props.tagCollection;
      var taskModel = this.props.model;

      function storeTags() {
        if (taskModel.isNew()) {
          taskModel.set({tag_ids: ids});
        } else {
          taskModel.save({tag_ids: ids}, {patch: true});
        }
      }

      var allTags = tagCollection.pluck('name');
      var newTags = _.difference(tags, allTags);

      var newTagModels = newTags.map(
        function createTagModel(tagName) {
          return new Tag.Model({name: tagName});
        }
      );

      var tagIds = tags.map(
        function findExistingTags(tag) {
          return tagCollection.where({name: tag});
        }
      );

      if (newTagModels) {
        var complete = _.invoke(tagCollection.add(newTagModels), 'save');
        $.when.apply($, complete).done(
          function finishCreatingTags(){
            var newTagIds = _.pluck(newTagModels, 'id');
            storeTags(_.union(tagIds, newTagIds));
          }
        );
      } else {
        storeTags(tagIds);
      }

    },