docs: function renderFilters (req, modelName, cb) {
  
      if (!req) {
          throw new Error('req is required.');
      }
  
      if (typeof modelName !== 'string' || !modelName.length) {
          throw new Error('modelName is required and must be of type String and cannot be empty.');
      }
  
      linz.api.model.list(req, modelName, function (err, list) {
  
          if (err) {
              return cb(err);
          }
  
          if (!Object.keys(list.filters).length) {
              return cb(null);
          }
  
          async.each(Object.keys(list.filters), function (fieldName, filtersDone) {
  
              // call the filter renderer and update the content with the result
              list.filters[fieldName].filter.renderer(fieldName, function (filterErr, result) {
  
                  if (filterErr) {
                      return filtersDone(filterErr);
                  }
  
                  list.filters[fieldName].formControls = result;
  
                  return filtersDone(null);
  
              });
  
          }, function (filterErr) {
  
              return cb(filterErr, list.filters);
  
          });
  
      });
  
  }