protected function _fieldsAreNull($entity, $source)
    {
        $nulls = 0;
        $schema = $source->getSchema();
        foreach ($this->_fields as $field) {
            if ($schema->getColumn($field) && $schema->isNullable($field) && $entity->get($field) === null) {
                $nulls++;
            }
        }

        return $nulls === count($this->_fields);
    }