foreach ($constraint->fields as $field => $constraints) {
            if (array_key_exists($field, $value)) {
                // cannot simply cast to array, because then the object is converted to an
                // array instead of wrapped inside
                $constraints = is_array($constraints) ? $constraints : array($constraints);

                foreach ($constraints as $constr) {
                    $walker->walkConstraint($constr, $value[$field], $group, $propertyPath.'['.$field.']');
                }

                unset($extraFields[$field]);
            } else {
                $missingFields[] = $field;
            }
        }