I think it does not make sense to execute query and load full entity if I just need to fetch id of proxy object and this proxy has FK id.
I have Client and Account entities.
Account table has a FK on Client. (id_client)
relationship - one-to-many:

@Entity
class Client {
  @OneToMany(mappedBy="client")
 private List<Account> accounts;
}

@Entity
class Account {
   @JoinColumn(name = "id_client")
    @ManyToOne(fetch = FetchType.LAZY)
    private Client client;
}  

Account account = em.find(Account.class, 2);
account.getClient().getId();

console output:

Hibernate:
select
account0_.id_account as id_accou1_0_0_,
account0_.AMOUNT as AMOUNT2_0_0_,
account0_.id_client as id_clien3_0_0_
from
ACCOUNT account0_
where
account0_.id_account=2

Hibernate:
select
client0_.ID_CLIENT as ID_CLIEN1_4_0_,
client0_.age as age2_4_0_,
client0_.create_date as create_d3_4_0_,
client0_.modify_date as modify_d4_4_0_,
client0_.NAME as NAME5_4_0_,
client0_.vip as vip6_4_0_
from
client client0_
where
client0_.ID_CLIENT=1

There is id_client column in Account table

id_account | amount | id_client

Account object has javassist proxy client object, that proxy client has JavassistLazyInitializer handler field and there is id field.

Why don't just retrieve that id from handler object???