How to define a grapqlObject

I am trying to define a grapqlobject with Juniper to deal with relationship in tables

'''pub struct Agenda {
pub id: i32,
pub address: Option,
pub case_id : i32

pub struct Case {
pub id: i32,
pub contract_serial_number: Option,

#[juniper::object]
implement Case{

pub fn id(&self) -> i32 {
self.id
}
pub fn contract_serial_number(&self) -> &str {
contract_serial_number
'''

How can I deal with the Option String ?
How can I build a graphql object with relationship between the Agenda table and the Case table

I haven't seen any example in the documentation

Thanks in advance for your help