java - Hibernate persist entity without fetching association object. just by id

Question

I have an simple association between 2 entities:

public class Car {

    ...

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;

    ...

}

and

public class User {

    @Id
    @GeneratedValue
    @Column(name = "user_id")
    private long userId;

    ...

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "user")
    private Set<Car> cars;

    ...

}

Then I get some user id from client. For example, userId == 5;
To save car with user I need to do next:

User user = ... .findOne(userId);  
Car car = new Car();
car.setUser(user);
... .save(car);

My question is:
Can I persist car record without fetching user?

Similarly like I would do by using native SQL query: just insert userId like string(long) in Car table.

With 2nd lvl cache it will be faster but in my opinion I don't need to do extra movements.

The main reason that I don't want to use native Query is because I have much more difficult associations in my project and I need to .save(car) multiple times. Also i don't want to manually control order of query executions.

If I use session.createSQLQuery("insert into .....values()") will the Hibernate's batch insert work fine?

Correct me if I'm wrong.

Thanks in advance!

UPDATE:

Actually the mapping is similar to:

There is @ManyToMany association between User and Car. But cross table is also an entity which is named, for example, Passanger. So the mapping is next:

public class User{
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "user", targetEntity = Passenger.class)
    private Set<Passenger> passengers;
}

Cross entity

@IdClass(value = PassengerPK.class)
public class Passenger {

    @Id
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;

    @Id
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "car_id")
    private Car car;

    ... other fields ...

}

Car entity:

public class Car {
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "car", targetEntity = Passenger.class, cascade = CascadeType.ALL)
    private Set<Passenger> passengers;
}

And the code:

List<User> users = ... .findInUserIds(userIds); // find user records where userId is IN userIds - collection with user Ids
Car car = new Car();       //initialization of car's fields is omitted
if (users != null) {
    car.setPassengers(new HashSet<>(users.size()));
    users.forEach((user) -> car.getPassengers().add(new Passenger(user, car)));
}
... .save(car);

Answer 1

"Can I persist car record without fetching user?"

Yes, that's one of the good sides of Hibernate proxies:

User user = entityManager.getReference(User.class, userId); // session.load() for native Session API  
Car car = new Car();
car.setUser(user);

The key point here is to use EntityManager.getReference:

Get an instance, whose state may be lazily fetched.

Hibernate will just create the proxy based on the provided id, without fetching the entity from the database.

"If I use session.createSQLQuery("insert into .....values()") will the Hibernate's batch insert work fine?"

No, it will not. Queries are executed immediately.