Question

试图从加入获取最新记录时遇到困难我有以下课程

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'

     }]
   }

此处将返回作者列表及其各自的图书。

问题是JPA如何帮助我根据发布的日期从集合中挑选最新的书籍。

Answer 1

如果您正在使用休眠，则可以使用@JoinFormula来实现此目的，以按日期映射最新记录。类似的东西：

@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;

Answer 2

我有类似的问题。另一个解决方案是使用@Where批注：

@OneToMany
@JoinColumn(name = "author_id", referencedColumnName = "id")
@Where(clause = "date_published = (SELECT MAX(book.date_published) " +
                                   "FROM book" +
                                   "where author_id = book.author_id)")
@OrderBy("datePublished Desc")
private List<Book> books;

我在堆栈上的信息：Get applications with their latest status only

Answer 3

如果您想为每位作者获取最后一本书，可以添加瞬态字段来获取它：

@Entity
    @Table(name = "author")
    public class Author {
    .......
    @Transient 
    private Book lastBook;

   @PostLoad
   private void setLastBook() {
  if(books==null || books,isEmpty())
     this.lastBook=null;
  }else{
    this.lastBook=books.get(0);
  }
}

或一对一地将其保存在db中，使用@PostPersist和@PostUpdate注释的相同方法。它会在上一本书中自动保存在db中

JPA @OneToMany从Join获取最新记录

3 个答案: