code: function(message) {
    return override(function() {
      if (!this._isToMany) {
        var modelName = this._modelClass.__identity__.__name__;
        var relationName = this._name;
        throw new Error(util.format('%s for non many-to-many through relation ' +
          '%s#%s.', message, modelName, relationName));
      }
      return this._super.apply(this, arguments);
    });
  }
docs: A convenience function for creating a through-only method override that will
  throw an exception if this is not a through relation configured as a simple
  many-to-many.
  
  @function HasMany~manyToManyOnly
  @param {String} message The error message prefix.
  @return {Function} The method.
  @see {@link HasMany~throughOverride}