Java – why does hibernate generate inserts and updates for the onetomany map

I try to use a small example to understand the one - to - many mapping in hibernate My product has a set of part's. This is my entity class:

Part. java

@Entity
public class Part {
    @Id
    @GeneratedValue
    int id;
    String partName;
    //Setters & Getters
}

Product. java

@Entity
public class Product {
    private String serialNumber;
    private Set<Part> parts = new HashSet<Part>();
    @Id
    public String getSerialNumber() {
        return serialNumber;
    }
    @OneToMany
    @JoinColumn(name = "PRODUCT_ID")
    public Set<Part> getParts() {
        return parts;
    }
    // Setter methods
}

Then I try to save some parts and products in my database and observe the following query generated by Hibernate:

Hibernate: insert into Product (serialNumber) values (?)
Hibernate: insert into Part (partName,id) values (?,?)
Hibernate: update Part set PRODUCT_ID=? where id=?

To add a record in the part table, hibernate will generate two DML operations - insert and update If a single insert command is enough to add records to a table, why does hibernate use insert and update in this case? Please explain

Solution

I know it's crazy, but I have the same question. Google brought me here, so after repairing, I think I should send an answer

If you make the connection column non empty and non updatable, hibernate will switch the insert / update method to direct insert. I don't think it is any way in your case:

@JoinColumn(name = "PRODUCT_ID",nullable = false,updatable = false)