code: function(object) {
        if (object instanceof ModelClass) {
          return object.url();
        }
        if (object instanceof ModelInstance || isFunc(object.$model)) {
          var model = object.$model();
          return expr(object, model.$config().identity + '.href').get() || model.url();
        }
        throw new Error('Could not get URL for ' + typeof object);
      }
docs: Extract URL from object, or use default URL