Duplicate key in Hibernate JPA OneTOMany save operation

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This is my Item class

@Entity
@Table(name = "item", schema = "demo_transaction")
@Transactional
public class Item {
    @Id
    Integer item_id;
    String itemName;
    Integer price;

    @OneToMany(cascade = CascadeType.ALL,fetch= FetchType.EAGER)
    List<Event> events;

Event class:

@Entity
@Table(name = "event", schema = "demo_transaction")
@Transactional
public class Event {
    @Id
    Integer event_id;
String eventName;

As shown Item can have multiple Events. In typical SQL an FK table would be like this:

item_id event_id
1       10
1       20
2       10 
2       20

But when I am trying to save these I am getting constrain violation.

    Event e1=new Event(10,"FirstEvent");
    Event e2=new Event(20,"SecondEvent");
List<Event> lse1=new ArrayList<>();
        lse1.add(e1);
        lse1.add(e2);
        Item item1 = new Item(1,"FirstItem",600,lse1);
List<Event> lse2=new ArrayList<>();
        lse1.add(e1);
        lse1.add(e2);
        Item item2 = new Item(2,"SecondItem",200,lse2);
repo.save(item1);repo.save(item2);

I understand that since eventId is the primary key and I am trying to save twice that's where the constraint getting violated but how would I achieve that FK table? Please help

one-to-many hibernate-onetomany spring-data-jpa jpa hibernate
1 Answers

@GeneratedValue(strategy = GenerationType.IDENTITY

Add above annotaion to the event_id and item_id. Then it wiil generate unique ids for you.

Then add @JoinColumn(name = "item_id") to event_id field and mappedBy = "item" to inside of OneToMany of the item_id field .Also have to add @ManyToOne for event_id field.

  //for Item change as folllows

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    Integer item_id;

    @OneToMany(cascade = CascadeType.ALL,fetch= FetchType.EAGER 
     ,mappedBy = "item")
    List<Event> events;
    
        // for Event change as follows

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    Integer event_id;


    @ManyToOne
    @JoinColumn(name = "item_id")
    Item item;
YJR0
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