code: function normalize(selections, next) {
  
          populations = _.map(populations, function (population) {
  
            return function (cb) {
  
              //obtain population selected fields
              const fields =
                (selections ? selections[population.path] : undefined);
              if (fields) {
                population = _.merge({}, population, { select: fields });
              }
  
              //parse path specific select
              if (population.select) {
                exports.select(population, function (error, select) {
                  cb(error, _.merge({}, population, { select: select }));
                });
              }
  
              //continue
              else {
                cb(null, population);
              }
  
            };
  
          });
  
          async.parallel(populations, next);
  
        }
docs: normalize populations