code: public List<BeanId> getReference(final String propertyName) {
          List<BeanId> values = references.get(propertyName);
          if (values == null) {
              return null;
          }
          return values;
      }
docs: Get the references of a property on a bean.
  
  @param propertyName name of the property as defined by the bean's schema.
  @return References that identify other beans.