code: public org.grails.datastore.mapping.query.api.Criteria sqlRestriction(String sqlRestriction) {
          if (!validateSimpleExpression()) {
              throwRuntimeException(new IllegalArgumentException("Call to [sqlRestriction] with value [" +
                      sqlRestriction + "] not allowed here."));
          }
          return sqlRestriction(sqlRestriction, Collections.EMPTY_LIST);
      }
docs: Applies a sql restriction to the results to allow something like:
  
  @param sqlRestriction the sql restriction
  @return a Criteria instance