code: function () {
      var sortValue = this.get("sortValue");
      if (_.isString(sortValue)) return this[sortValue];
      else if (_.isFunction(sortValue)) return sortValue;
  
      return function (model, colName) {
        return model.get(colName);
      };
    }
docs: Returns an appropriate value extraction function from a model for sorting.
  
  If the column model contains an attribute `sortValue`, if it is a string, a
  method from the column instance identifified by the `sortValue` string is
  returned. If it is a function, it it returned as is. If `sortValue` isn't
  found from the column model's attributes, a default value extraction
  function is returned which will compare according to the natural order of
  the value's type.
  
  @return {function(Backbone.Model, string): *}