code: public BeanId getFirstReference(final String propertyName) {
          List<BeanId> refrences = getReference(propertyName);
          if (refrences == null || refrences.size() < 1) {
              return null;
          }
          return refrences.get(0);
      }
docs: A helper method for getting the value of single referenced property. Returns
  null if the refrences does not exist.
  
  @param propertyName name of the property as defined by the bean's schema.
  @return the value.