java FindBy 在 JPA 中使用外键
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FindBy using a foreign key in JPA
提问by joe11093
I have the 3 following Entities in my project.
我的项目中有以下 3 个实体。
@Entity
public class Review {
@Id
@GeneratedValue
private int reviewId;
@OneToMany(mappedBy="review", cascade = CascadeType.ALL)
private List<Comment> comments;
@ManyToOne
private User user;
}
@Entity
public class Comment {
@Id
@GeneratedValue
private int commentId;
@ManyToOne
private Review review;
@ManyToOne
private User user;
}
@Entity
public class User {
@Id @GeneratedValue
private Long userId;
@OneToMany(mappedBy="user", cascade = CascadeType.ALL)
private List<Comment> comments;
@OneToMany(mappedBy="user", cascade = CascadeType.ALL)
private List<Review> reviews;
}
I want to use JPA to fetch every Commenton a particular Review, such that under the page of each Review, I can display the name of the Userwho commented, along with the actual Comment. So when I visit the page http://localhost:8080/review/view/5, I want to be able to see the review in addition to all comments made on it, along with the names of the users who added the comments.
我想使用JPA获取每Comment一个特定的Review，使得每个页面下Review，我可以显示的名字User谁评论说，与实际一起Comment。因此，当我访问该页面时http://localhost:8080/review/view/5，除了对其发表的所有评论之外，我还希望能够看到评论，以及添加评论的用户的姓名。
Is this achievable without writing the SQL myself? If yes, how?
这是否可以在不自己编写 SQL 的情况下实现？如果是，如何？
回答by Maciej Kowalski
Use an entity graph in your spring data jpa Repository:
在您的 spring 数据 jpa 存储库中使用实体图：
@EntityGraph(value = "Review.comments", type = EntityGraphType.FETCH)
public Review findByReviewId(int id);
or explicitly define a @Query:
或者明确定义一个@Query：
@Query("from Review r inner join fetch r.comments where r.reviewId = :id")
User findByReviewId(@Param("id") int id);
