I only find modules or answers for v6 and v8.
Can someone help me on v7?

class stock_picking_out(orm.Model):
    _inherit = 'stock.picking.out'  
    
    def _get_sale_client_order_ref(self, cr, uid, ids, field_name, arg, context={}):
        res = {}
        for session in self.browse(cr,uid,ids,context=context):
            if (session.sale_id):
                res[session.id] = session.sale_id.client_order_ref or False
        return res
           
    _columns = {
        "client_order_reference": fields.function(_get_sale_client_order_ref, type='char', size=64,
                                                 method=True, string="Sales Order Customer Reference"
                                                 )
    }