Thanks for the quick answer, i'm afraid i don't understant where to put the "declaration" of my new fields :

$object->data->additional_field_one = '123';
$object->data->additional_field_two = '234';

and for the corresponding update function ? do i update the "data" field ?

public function updateMemo($title ='', $text = '') {
     ...
    $params['names']  = array(
        '??????',   // what goes here to update $memo->data->lastupdate
        'title',
        'description'
    );
    $params['values'] = array(
        time(),
        $title,
        $text

    );
    ...
}

EDIT:
found out i can use the save() to update
EDIT 2:
I'm still struggling with the declaration part though

Here you go

$object->data->additional_field_one = '123';
$object->data->additional_field_two = '234';

Once this created you may access directlyusing echo $memo->additional_field...

Thank you both for your answers, i made my Memo plugin basing it on the blog example and it works ! I made my class Memo extend OssnObject and stored the title in title and the text in description.

Now i'd like to add additional informations on my memos : a list of tags and a separate entry for the last edition date and the creation date.

Where would you put such additional information as OssnObject only have limited number of fields ?

There are several existing tables you can use to hold your data if you don't want to modify the db schema by adding a table or a column to an existing table. As Arsalan pointed out, the Blog component will show you how to use some of these. The tables you'd potentially want to look at are ossnentities, ossnentitiesmetadata, ossnobject and maybe ossn_annotations. The other component that might be useful to look at is the Files component if you have that.

Best example is : https://www.opensource-socialnetwork.org/component/view/1653/blog (you can remove some of functions like public blogs etc) Please take a look if you didn't understand let us know.