protected static function addRelationOneToMany($sourceField, $classRelation, $targetField, $aliasRelation = '')
    {
        // test is related class is a PicORM model
        if (!class_exists($classRelation) || !new $classRelation() instanceof Model) {
            throw new Exception("Class " . $classRelation . " doesn't exists or is not subclass of PicORM\Model");
        }

        // create relation id with normalized classRelation name
        $idRelation = self :: formatClassnameToRelationName($classRelation);

        // override the relation's id if an alias is present
        if (!empty($aliasRelation)) {
            $idRelation = strtolower($aliasRelation);
        }

        // store new relation in model
        static::$_relations[$idRelation] = array(
            'typeRelation'  => self::ONE_TO_MANY,
            'classRelation' => $classRelation,
            'sourceField'   => $sourceField,
            'targetField'   => $targetField,
        );
    }