Fetching & Updating lazy-loaded many-many fields in Spring Data REST

问题

How do I correctly expose lazy-loaded many-many fields so that users can GET/PATCH/POST/DELETE many-many entity relationships in Spring Data REST?

For example, given a Student entity and Teacher entity bound by a many to many relationship, with the following POJOs:

@Entity
public class Teacher { // owner of bidirectional relationship
    @Id
    private int id;
    private String name;
    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "teacher_student",
            joinColumns = @JoinColumn(name = "teacher_id"),
            inverseJoinColumns = @JoinColumn(name = "student_id"))
    private Set<Student> students;

    // Constructor, getters/setters...
}

@Entity
public class Student {
    @Id
    private int id;
    private String name;
    @ManyToMany(mappedBy = "students", fetch = FetchType.LAZY)
    private Set<Teacher> teachers;

    // Constructor, getters/setters...
}

The entities are given repositories:

@RepositoryRestResource(path = "teacher")
public interface TeacherRepository extends CrudRepository<Teacher, int> {}

// similar repository for student

When I send a GET to localhost:8080/teacher, I get:

"_embedded": {
    "teacher": [
        {
        "name": "Bill Billie",
        "_links": {
            "self": { "href": "http://localhost:8080/teacher/1" },
            "teacher": { ... },
            "students": { "href": "http://localhost:8080/teacher/1/students" }
        }},
        (more teachers here...)
    ]
}
...

BUT, when I try a GET to http://localhost:8080/teacher/1/students, I get a 404 Not Found, even though the teacher "Bill Billie" does have a student associated with him in the database.

Interestingly, if I change the FetchType to FetchType.EAGER, everything works fine and I can perform the expected GET, PATCH, etc. What gives? Is this a bug, perhaps, or am I screwing something up?

tl;dr Many-many relations are not correctly exposed with lazy fetching, but work fine with eager fetching. How can I get lazy fetching to work with it?

Edit: If it matters, I am using Spring 4.2.6 with Spring Boot 1.3.5, and OpenJPA 2.4.1 as my JPA provider.

回答1:

Hmm I am not sure why it isn't auto-fetching someone more experienced would have to ask that, but you can specify a manual fetch with HQL's join fetch for each query.

select x from X left join fetch x.y y

After getting this working you can override your get statement with a specifically created controller as described here: Spring Data REST: Override repository method on the controller

Only other option I think might be worth trying is adding an @Lazy annotation on your Spring-data repository.

回答2:

Try making your RestResources transactional.

Annotate with @Transactional

回答3:

Add the following dependency

<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-hibernate4</artifactId>
</dependency>

Add the configuration class.

@Configuration
@EnableWebMvc
public class WebConfig extends WebMvcConfigurerAdapter{


    public MappingJackson2HttpMessageConverter jacksonMessageConverter(){
        MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();

        ObjectMapper mapper = new ObjectMapper();
        //Registering Hibernate4Module to support lazy objects
        mapper.registerModule(new Hibernate4Module());

        messageConverter.setObjectMapper(mapper);
        return messageConverter;

    }

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        //Here we add our custom-configured HttpMessageConverter
        converters.add(jacksonMessageConverter());
        super.configureMessageConverters(converters);
    }
}

来源：https://stackoverflow.com/questions/37887733/fetching-updating-lazy-loaded-many-many-fields-in-spring-data-rest

标签

java

Spring

spring-data-jpa

openjpa

spring-data-rest