code: function ( req ) {
  
      var pk = req.options.id || (req.options.where && req.options.where.id) || req.param('id');
  
      // TODO: make this smarter...
      // (e.g. look for actual primary key of model and look for it
      //  in the absence of `id`.)
      // See coercePK for reference (although be aware it is not currently in use)
  
      // exclude criteria on id field
      pk = _.isPlainObject(pk) ? undefined : pk;
      return pk;
    }
docs: Parse primary key value for use in a Waterline criteria
  (e.g. for `find`, `update`, or `destroy`)
  
  @param  {Request} req
  @return {Integer|String}