code: function ensureID(options, prop, callback) {
    if(options[prop]) {
      return callback();
    }
  
    options.guid(function(err, id) {
      if(err) {
        return callback(err);
      }
      options[prop] = id;
      callback();
    });
  }
docs: Make sure the options object has an id on property,
  either from caller or one we generate using supplied guid fn.