Hi,

This is an example of stage_id many2one field

stage_id = fields.Many2one('maintenance.stage', string='Stage', ondelete='restrict', tracking=True,
                           group_expand='_read_group_stage_ids', default=_default_stage, copy=False)


@api.model
def _read_group_stage_ids(self, stages, domain, order):
    """ Read group customization in order to display all the stages in the
        kanban view, even if they are empty
    """
    stage_ids = stages._search([], order=order, access_rights_uid=SUPERUSER_ID)
    return stages.browse(stage_ids)

Regards