See the below example

In Python:

  _columns = {
            'name': fields.many2one('hr.employee', "Employee", required=True),
            'id_no': fields.char('Wage', size=64),
        }

        def onchange_employee_id(self, cr, uid, ids, employee_id, context=None):
            v = {}
            if employee_id:
                emp_sr = self.pool.get('hr.employee').search(cr, uid, [('id','=',employee_id)], context=context)
                if emp_sr:
                    empt_br = self.pool.get('hr.employee').browse(cr, uid, emp_sr, context=context)[0]
                    v['id_no'] = str(emp_br.id)

            return {'value': v}

In XML:

<field name="name" on_change="onchange_employee_id(name)"/>
<field name="id_no"/>