JPA加入继承的类

Question

In jpa I have some entity ( Content and Product for now but more can come) that should join to Comment entity.

Problem is I don't want to have extra field(and column in table) in Comment entity for each join entity (Products, Contents..) because these entities will increase in future.

I find one semi-solution is to use single table inheritance and create concrete Comment class like CommentContent , CommentProduct , and use discriminator column but joining to entities (Content and Product) still remain.

what do you suggest?

Edit:

sample relation for example between Comment and Content will be @MayToOne that many Comments belongs to one Content and so for Product..

Edit 2

in pure table relationship schema (without ORM like hibernate/jpa) I can and I do this kind of solution: add to column in comment table 1-item_type and 2-item_id witch item_type specify other side table name (product, content in my question) and item_id specify foreign key to table that it name is at item_type column

How can I model this in Jpa/hibernate ORM?

Answer 1

You can model what you described with Hibernate like

class Content {
  @OneToMany
  @JoinColum(name = "item_id")
  @Where("item_type = 'CONTENT'")
  Set<Comment> comments;
}

class Product {
  @OneToMany
  @JoinColum(name = "item_id")
  @Where("item_type = 'PRODUCT'")
  Set<Comment> comments;
}

class Comment {
  @Id
  @GeneratedValue
  Long id;
  @Enumerated(STRING)
  ItemType itemType;
  Lont itemId;
}

enum ItemType {
  CONTENT,
  PRODUCT
}

Answer 2

What type or relation is this?

If it is @OneToOne you can put information about relation with Comment in Product and Content, and in Comment use option MappedBy to map them, you will skip the extra columns in Comment.

If it is @OneToMany you won't make any column in Comment but rather again put information in Product and Content and only use mappedBy on collection of this objects on the Comment site.

CheckForMoreInfo

Answer 3

You can try @ManyToOne for your upcoming entities. So here you don't need to add new columns in Comment entity(base table) Check this example : https://howtoprogramwithjava.com/hibernate-manytoone-unidirectional-tutorial/