即使设置了FetchType.EAGER，Spring也不会加载数据 [英] Spring not loading data even with FetchType.EAGER set

问题描述

我有两个模型，我正在尝试从REST API(宠物和媒体)获取数据.我试图通过FetchType.EAGER批注获取宠物和急切加载的媒体之间的oneToMany关系，但是在编写MediaRepository时没有出现数据.如果我没有实现该文件，则媒体关系和数据会在响应中返回.

I have two models I'm trying to get data back from a REST API (Pet and Media). I'm trying to get the oneToMany relationship between pet and media eagerly loaded via the FetchType.EAGER annotation, but the data doesn't appear when I write the MediaRepository. If I don't implement that file, the media relationship and data comes back in the response.

实现MediaRepository.java后，GET/pets返回:

With MediaRepository.java Implemented, GET /pets returns:

{
  "id": 72,
  "name": "Spot",
  "description": "Annoying as hell",
  "media": [], <-- why is this here only if I don't implement MediaRepository?
  ...
}

没有实现MediaRepository.java，GET/pets返回:

Without MediaRepository.java Implemented, GET /pets returns:

{
  "id": 72,
  "name": "Spot",
  "description": "Annoying as hell",
  ... (No media array in response)
}

Pet.java

@Entity
public class Pet implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name="name")
    private String name;

    @Column(name="description")
    private String description;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="pet", FetchType.EAGER, orphanRemoval=true)
    private List<Media> media;

    @ManyToOne
    private Category category;

    @Enumerated(EnumType.STRING)
    private Status status;
}

Media.java

@Entity
public class Media implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name="url")
    private String url;

    @Column(name="title")
    private String title;

    @ManyToOne
    private Pet pet;
}

PetRepository.java

import org.springframework.data.jpa.repository.JpaRepository;

public interface PetRepository extends JpaRepository<Pet, Long> {
}

MediaRepository.java

import org.springframework.data.jpa.repository.JpaRepository;

public interface MediaRepository extends JpaRepository<Media, Long> {
}

推荐答案

这与Hibernate提取策略无关.

This has nothing to do with Hibernate fetch strategy.

您看到的行为是Spring Data Rest设计的工作方式.为媒体定义存储库后，您将看到响应中提供了一个链接，供客户端检索关联的媒体项.没有存储库，则必须在响应中插入关联，因为当然没有办法独立地检索集合.

The behavior you are seeing is how Spring Data Rest is designed to work. When you have defined a Repository for Media then you will see that a link is provided in the response for clients to retrieve the associated Media items. Without the repository then the association has to be in-lined in the response as there is of course no means to retrieve the collection independently.

如果您希望在响应中选择性地内联集合，则可以通过定义投影来实现.

If you wish to selectively in-line collections in a response then you can do by defining a Projection.

@Projection(name = "inlineData", types=Pet.class)
public interface PetProjection{

    Long getId();
    String getName();
    String getDescription();
    List<Media> getMedia();
}

您可以将此投影自动应用到集合资源:

You can have this projection applied automatically to a collection resource:

@RepositoryRestResource(excerptProjection = PetProjection.class)
public interface PetRepository extends JpaRepository<Pet, Long> {}

对于项目资源，客户通常会指定他们希望内联此数据:

For item resources client would typically specify that they want this data in-line:

例如

http://example.com/api/pets/1?projection=inlineData

http://docs.spring.io/spring-data/rest/docs/current/reference/html/#projections-excerpts

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