about extends ModuleAdminController change helper list position

[langziyang]

hi,i have module, there is admin controller file extends ModuleAdminController.i add position for it

protected $position_identifier = 'id_et_indexcategory_to_move';

'position' => array(
                'title'    => $this->trans('Position', array(), 'Admin.Global'),
                'position' => 'position',
                'align'    => 'center',
            ),

the BO like this:

 

when i change the position, i do not get the id_et_indexcategory_to_move value

so i can not change position,how can i get it

 

i use prestashop1.7