code: function(iteratee, instance) {
      if (_.isFunction(iteratee)) return iteratee;
      if (_.isObject(iteratee) && !instance._isModel(iteratee)) return modelMatcher(iteratee);
      if (_.isString(iteratee)) return function(model) { return model.get(iteratee); };
      return iteratee;
    }
docs: Support `collection.sortBy('attr')` and `collection.findWhere({id: 1})`.