unable to edit content in admin controller

[bonsaiko]

I developped my own module and I'm trying to edit content from the admin side.

In prestashop back office I was able to add a new menu item. 

When I click on it, it displays the list of the elements of my table, that's ok. 

When I click on an item, it shows me the item details. This is fine too.

But when I click on the edit button, I can see a form, but it's empty, instead of displaying the details of the item, to let me make some changes for example if I need to edit it.

Any idea of why I'm having this issue ?

I added these lines in my admin controller constructor:

$this->fields_form = array(
    'legend' => array(
        'title' => 'Ajouter / éditer auteur'
    ),
    'input' => array(
        array('type' => 'text', 'label' => $this->l('Firstname'), 'name' => 'firstname', 'size' => 30, 'required' => true),
        array('type' => 'text', 'label' => $this->l('Lastname'), 'name' => 'auteur_lastname', 'size' => 30, 'required' => true),
    ),
    'submit' => array('title' => 'Enregistrer')
);

I thought it would be enough to have things work but it seems not to be the case...

So form is correctly displayed, but fields are empty.

Any help would really be appreciated as I'm stuck. 

I was following a tutorial and I did exactly as it was explained, but I think Prestashop 1.7 must be different from 1.6 and that's why its not working.