spring - JPA @OneToMany 从 Join 获取按日期的最新记录

Question

试图从 获取最新记录时卡住了加入
我有以下类(class)

作者

    @Entity
    @Table(name = "author")
    public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    @Column(name = "name")
    private String name;
    @OneToMany
    @JoinColumn(name = "author_id", referencedColumnName = "id")
    @OrderBy("id Desc")
    private List<Book> books;
    public int getId() {
       return id;
    }
    public void setId(int id) {
       this.id = id;
    }
    public String getName() {
       return name;
    }
    public void setName(String name) {
       this.name = name;
    }
    public List<Book> getBooks() {
       return books;
    }
    public void setBooks(List<Book> books) {
       this.books = books;
     }
    }

书

    @Entity
    @Table(name = "book")
    public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    @Column(name = "author_id")
    private Integer authorId;
    @Column(name = "date_published")
    private Date datePublished;
    public int getId() {
        return id;
    }
    public void setId(int id) {
       this.id = id;
    }
    public Integer getAuthorId() {
       return authorId;
    }
    public void setAuthorId(Integer authorId) {
       this.authorId = authorId;
    }
    public Date getDatePublished() {
       return datePublished;
    }
    public void setDatePublished(Date datePublished) {
       this.datePublished = datePublished;
     }
    }

存储库

    @Repository
    public interface AuthorRepository extends 
    JpaRepository<Author, Long> {

         public Page<Author> findALL(int id, Pageable pageable);

    }

当前结果

    {
      "id": 1,
      "name": "James",
      "books":[
      {
         "id": 1,
         "name": "book1",
         "datePublished": '12/12/2012'

      },
      {
         "id": 1,
         "name": "book2",
         "datePublished": '01/02/2013'
      }]
    },

    {
      "id": 2,
      "name": "Tim",
      "books":[
      {
         "id": 5,
         "name": "book5",
         "datePublished": '12/12/2014'

      },{
          "id": 6,
          "name": "book6",
          "datePublished": '01/02/2015'

       }]
    }

预期结果

    {
       "id": 1,
       "name": "James",
       "books":[
       {
         "id": 1,
         "name": "book2",
         "datePublished": '01/02/2013'
       }]
   },
   {
     "id": 2,
     "name": "Tim",
     "books":[
     {
        "id": 6,
        "name": "book6",
        "datePublished": '01/02/2015'

     }]
   }

从这个列表 作者 正在归还他们各自的所有书籍。

问题是 JPA 如何帮助我根据出版日期仅从收藏中挑选最新的书。

Answer 1

如果您正在使用休眠，您可以使用 @JoinFormula 来实现这一点。按日期映射最新记录。就像是:

@ManyToOne(fetch = FetchType.LAZY)
@JoinFormula("(" +
    "SELECT b.id " +
    "FROM book b " +
    "WHERE b.author_id = id " +
    "ORDER BY b.date_published DESC " +
    "LIMIT 1" +
")")
private Book latestBook;