code: public String validateSimpleUnique(Identifiable<?> entity, String property, Object value) {
          Map<String, Object> values = newHashMap();
          values.put(property, value);
          return existsInDatabaseOnAllObjects(entity, values) ? simpleUniqueConstraintError(entity, property) : null;
      }
docs: /*
  Return the error code if the given property is already present in the database, returns null otherwise.