elgg_get_entities_from_metadata, can't get my metadatas

Hi,

I save an object with metadatas:
//...etc.

        $my_obj->title = $title;
        $my_obj->my_obj_type = $type;
        $my_obj->my_obj_category = $category;
        $my_obj->description = $description;


    // Now save the object
        if (!$my_obj->save()) {
            register_error(elgg_echo("my_obj:error"));
            forward($_SERVER['HTTP_REFERER']);
        }

//...etc.

 

Then I want to get the metadata from the database with elgg_get_entities_from_metadata

But I get just the title and the description, and there is no my_obj_type or my_obj_category!
Why???

//My code about the query:

$params = Array (
    "types" => "object",
    "subtypes" => "my_obj",
    "limit" => 100,
    "order_by" => "time_updated DESC",
    "metadata_name_value_pairs" => array (
                array("name" => 'my_obj_type'),
                array("name" => 'my_obj_category')

    )
);

$my_obj = elgg_get_entities_from_metadata($params);

And what I get on print_r($my_obj);

Array
(
    [0] => ElggObject Object
        (
            [attributes:protected] => Array
                (
                    [guid] => 89
                    [type] => object
                    [subtype] => 11
                    [owner_guid] => 4
                    [container_guid] => 4
                    [site_guid] => 1
                    [access_id] => 1
                    [time_created] => 1317631059
                    [time_updated] => 1317631059
                    [last_action] => 1317631059
                    [enabled] => yes
                    [tables_split] => 2
                    [tables_loaded] => 2
                    [title] => Title of my object
                    [description] => Some text here :)
                )

            [url_override:protected] => 
            [icon_override:protected] => 
            [temp_metadata:protected] => Array
                (
                )

            [temp_annotations:protected] => Array
                (
                )

            [volatile:protected] => Array
                (
                )

            [valid:ElggEntity:private] => 
        )
)

How can I get my other metadatas?

Thanks for the help!