Hibernate ManyToOne vs OneToOne

Question

I can't see any difference in the schema of a Many-To-One relationship vs a OneToOne relationship:

@Entity
public class Order {

    @ManyToOne
    @JoinColumn(nullable = false)
    private Address address;

vs

@Entity
public class Order {

    @OneToOne
    @JoinColumn(nullable = false)
    private Address address;

Is there any difference?

score 61 · Accepted Answer · answered Aug 27 '13 at 11:09

61

They look exactly the same on schema but there is difference on Hibernate Layer.

If you try something like that:

Address address = new Address();
Order order1 = new Order();
order1.setAddress(address);
Order order2 = new Order();
order2.setAddress(address);
save();

Everything will be OK. But, after save if you try get Order:

@OneToOne case:
org.hibernate.HibernateException: More than one row with the given identifier was found: 1

@ManyToOne case:
SUCCESS

Of course, your Address class should looks different in both cases.

answered Aug 27 '13 at 11:09

paulek

883
7
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So I should be fine to switch between the two mappings? – DD. Aug 28 '13 at 05:54
1

What do you mean by "switch between the two mappings"? If you have existing schema of database and you decide to change mapping annotation in your application you can do it without changing the schema (from @OneToOne to @ManyToOne). In other way you can also do it, but you can have problem with data. – paulek Aug 28 '13 at 06:55
9

For anybody reading paulek's post in the future: Your entitites have to be _just so_ to get that error. If you have unidirectional relations only, that error will not pop up. If you have bidirectional relations, only a select on adresses with join fetching the orders will generate that error. – Martin Försterling Mar 10 '17 at 10:02
1

Why isn't the exception actually generated while `save()` in the OneToOne case? – sql_dummy Feb 14 '18 at 14:45
I tried save as `session.save(order1)` follwed by `session.save(order2)` and get an exception ` org.hibernate.PropertyValueException: not-null property references a null or transient value ` – sql_dummy Feb 15 '18 at 10:24
Also can you tell which object calls `save();` here. – sql_dummy Feb 15 '18 at 10:26

score 8 · Answer 2 · answered Aug 27 '13 at 08:25

8

There should normally be a unique constraint on the address_id join column in the case of a OneToOne association, to guarantee that only one Order can have a given address.

answered Aug 27 '13 at 08:25

JB Nizet

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1

This doesn't appear to be the case – DD. Aug 28 '13 at 05:55
At least with MySQL it creates the foreign key with an index of "Unique" type. – borjab May 26 '15 at 14:51

agoldev · Answer 3 · 2020-07-06T06:43:07.297

TL;DR: The schema might look the same at first glance, but the OneToOne may have an additional UNIQUE INDEX constraint. This guarantees the OneToOne association.

This is nicely illustrated by the Doctrine ORM documentation of Association Mapping (I don't think it's specific to Hibernate).

An example:

ManyToOne:

Consider the tables User and Address while the column User.address_id has a ManyToOne association to the Address.id column. This would be the SQL:

CREATE TABLE User (
    id INT AUTO_INCREMENT NOT NULL,
    address_id INT DEFAULT NULL,
    PRIMARY KEY(id)
) ENGINE = InnoDB;

CREATE TABLE Address (
    id INT AUTO_INCREMENT NOT NULL,
    PRIMARY KEY(id)
) ENGINE = InnoDB;

ALTER TABLE User ADD FOREIGN KEY (address_id) REFERENCES Address(id);

OneToOne:

Now, consider the tables Product and Shipment, while the column Product.shipment_id has a OneToOne (unidirectional) association to the Shipment.id column. This would be the SQL:

CREATE TABLE Product (
    id INT AUTO_INCREMENT NOT NULL,
    shipment_id INT DEFAULT NULL,
    UNIQUE INDEX UNIQ_6FBC94267FE4B2B (shipment_id),
    PRIMARY KEY(id)
) ENGINE = InnoDB;
CREATE TABLE Shipment (
    id INT AUTO_INCREMENT NOT NULL,
    PRIMARY KEY(id)
) ENGINE = InnoDB;
ALTER TABLE Product ADD FOREIGN KEY (shipment_id) REFERENCES Shipment(id);

The only difference is the UNIQUE INDEX directive which dictates there must not be a shipment.id occuring twice in the Product table. This guarantees the OneToOne association.

score 0 · Answer 4 · edited Jan 06 '18 at 09:51

0

This indicates both sides of the relationship are best served by having the same primary key. It makes more sense this way since both sides are associated one and only one order in the case of the @OneToOne relationship.

edited Jan 06 '18 at 09:51

Vlad Mihalcea

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answered Aug 31 '17 at 13:14

Progster219

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Hibernate ManyToOne vs OneToOne

4 Answers4

ManyToOne:

OneToOne:

Linked