Hibernate persisting Set of @Embeddable objects throws exception

Question

My classes look simiar to this: (Offer class)

@Entity
public class Offer {
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private int id;
    @ElementCollection(fetch = FetchType.LAZY)
    private Set<Product> products;

    ...other fields
}

and Product class:

@Embeddable
public class Product {
    private String name;
    private String description;
    private int amount;
}

The problem is when I try to persist the Offer entity and try to add two objects to Offer's Set:

Product product = new Product("ham", "description1", 1);
Product product = new Product("cheese", "description2", 1);

I receive exception:

Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "offer_products_pkey"
  Details: Key (offer_id, amount)=(1, 1) already exists.

I have no idea why can't I persist two embeddable objects in the Set when one of them has the same value of "amount" field? Is it treated as ID somehow?

Maybe I should not create list of embeddable objects as it is not designed to be used like this? If so - then what if I dont need an entity of Product but want to keep set of it in another entity (Offer)?

Thanks in advance for help

You have probably wrong defined key in table Products. Exception is clear — mariusz2108, May 02 '16 at 16:46
I dont have any key variable in Product object. If I use List instead of Set it works fine, but for Set it does not. Problem seems to be bound up with features of Set interface - it cannot contain duplicates. But why is field "amount" treated as duplicate? — azalut, May 02 '16 at 16:52
The error comes from PostgreSQL. It tells you what is wrong: you have defined what looks like a primary key named offer_products_pkey for (offer_id, amount). And you're trying to insert two rows with the same primary key. drop that primary key constraint if it shouldn't exist. — JB Nizet, May 02 '16 at 18:18

score 0 · Answer 1 · answered May 05 '16 at 14:45

The issue when using a Set is that the contents must be unique, since that is the definition of a Set. The JPA provider will try to enforce this with a database constraint. In your example it adds a constraint in the form of a Primary Key the Offer_id and the int Amount, though IMHO it should be adding a constraint for all the values of Product property. The best way to see this is to enable the SQL logs and see what is going on under the covers:

Hibernate: create table Offer (id integer not null, primary key (id))
Hibernate: create table Offer_products (Offer_id integer not null, amount integer not null, description varchar(255), name varchar(255), primary key (Offer_id, amount))
Hibernate: alter table Offer_products add constraint FKmveai2l6gf4n38tuhcddby3tv foreign key (Offer_id) references Offer

The way to fix this is to make the products property of Offer a List instead of the Set:

Hibernate: create table Offer (id integer not null, primary key (id))
Hibernate: create table Offer_products (Offer_id integer not null, amount integer not null, description varchar(255), name varchar(255))
Hibernate: alter table Offer_products add constraint FKmveai2l6gf4n38tuhcddby3tv foreign key (Offer_id) references Offer

Hibernate persisting Set of @Embeddable objects throws exception

1 Answers1