code: function(req) {
  
      var pk = module.exports.parsePk(req);
  
      // Validate the required `id` parameter
      if (!pk) {
  
        var err = new Error(
          'No `id` parameter provided.' +
          '(Note: even if the model\'s primary key is not named `id`- ' +
          '`id` should be used as the name of the parameter- it will be ' +
          'mapped to the proper primary key name)'
        );
        err.status = 400;
        throw err;
      }
  
      return pk;
    }
docs: Parse primary key value from parameters.
  Throw an error if it cannot be retrieved.
  
  @param  {Request} req
  @return {Integer|String}