[FIXED] JPA Hibertane unidirectional one-to-one with shared primary key save parent first NO REVERSE

Issue

I'v been searching internet for answer, but nothing was working for me. There is a lot of topics with similar cases but specific details are different in a way that make them unusable for me.

So I have two tables: t_item and t_item_info:

item_id field from t_item_info table references id field from t_item table. I'm using mysql db and id column form t_item is auto incremented

I need to make unidirectional one-to-one mapping in a specific way. Here are my classes:

@Entity
@Table(name = "t_item")
public class Item {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY, optional = false)
    @PrimaryKeyJoinColumn(name = "id", referencedColumnName = "item_id")
    private ItemInfo info;
}

And other one

@Entity
@Table(name = "t_item_info")
public class ItemInfo {
    @Id
    private Long itemId;

    private String descr;
}

So the point is that i need Item object to have a reference to ItemInfo object. NOT The other way!

Item -> ItemInfo   --YES
Item <- ItemInfo   --NO

The other thing is that i need parent (Item) id to become id for a child (ItemInfo)

For example I create Item object with null id and set it's info field with ItemInfo object which also have null id field. Like this:

{
  "id": null,
  "name": "Some name",
  "info": {
    "itemId": null,
    "descr": "some descr"
  }
}

Then when Item object persists hibernate should generate id for parent(Item) object and set it as itemId field for child(ItemInfo).

I have been trying to achieve this with different hibernate annotations and I noticed that no matter how hard I tried Hibernate always seems to try to persist child object first. I noticed it in the logs when I turned sql logging on. insert into t_item_info always goes first (and dies because of null id :D)

So the question is: Is it even possible to achieve this and if so what should I change in my code to do so

I hope that what I'm trying to ask makes sens to you given my poor explanations =)

Solution

Why people always insist the child object table in one-to-one associations should be the one with the foreign key is beyond me.

Anyway, as a workaround, since both objects share the id and the association is non-optional, you might as well declare the autogenerated key for the child object:

@Entity
@Table(name = "t_item_info")
public class ItemInfo {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long itemId;

    private String descr;
}

and then use @MapsId for the parent:

@Entity
@Table(name = "t_item")
public class Item {
    @Id
    private Long id;

    private String name;

    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "id")
    @MapsId
    private ItemInfo info;
}

Note that this approach, will, in a sense, fool Hibernate into thinking it is the Item that should be treated as the child object. You have been warned.

Answered By - crizzis