protected function getConditionStringFromBelongsTo(BelongsTo $relation)
    {
        $id = $this->getAttribute( $relation->getForeignKeyName() );

        // an empty foreign key will, as a null-scope, remove the item from any list
        if (null === $id) return null;

        return '`' . $relation->getForeignKeyName() . '` = ' . (int) $id;
    }