JPA更新一对多关系列表 [英] JPA update list of one to many relationship

问题描述

  public class Question {
 @OneToMany（fetch = FetchType.LAZY，mappedBy =question，cascade = CascadeType.ALL）
 @JsonManagedReference 
 private List< Alternative> alternativeList; 
 
 $ b $ public class Alternative实现Serializable {
 @ManyToOne（fetch = FetchType.LAZY）
 @JoinColumn（name =questionId，nullable = false）
 @JsonBackReference 
私人问题问题; 
 
} 
  

然后我想用新的替代列表的一套。为此，我使用新的Question对象调用JpaDao元素的合并方法：

  @Repository 
 public class JpaQuestionDao实现了QuestionDao {
 @PersistenceContext 
 private EntityManager em; 
 
 @Transactional 
 public问题更新（问题）{
 return em.merge（question）; 
} 
} 
  

然而，这实际上是合并了两个列表：已经在数据库中的一个和新提供的一个。我有这样的方法没有任何问题，当我有非列表对象，这就是为什么我一直使用合并方法。

有没有替代方法不合并，但只更新列表？

解决方案

首先使用EntityManager.find（）检索问题对象。问题对象将作为结果进行管理。由于alternativeList是延迟加载的，因此您必须调用collections对象上的一个方法来获取它并进行管理。对问题对象和列表的任何更新都会自动发送到数据库。

  @Transactional 
 public Question更新（问题）{
问题q = em.find（Question.class，question.getId（））; 
 q.setAlternativeList（null）; 
 q.setAlternativeList（question.getAlternativeList（））; 
 ... 
 
 return q; 
} 
  

或者，您可以尝试对@OneToMany集合使用orphanRemoval = true

  @OneToMany（fetch = FetchType.LAZY，mappedBy =question，cascade = CascadeType.ALL，orphanRemoval = true）
 @JsonManagedReference 
 private List< Alternative> alternativeList; 
  

I have a Question entity with a list of another entity called Alternatives like this:

public class Question {
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "question", cascade = CascadeType.ALL)
    @JsonManagedReference
    private List<Alternative> alternativeList;
}

public class Alternative implements Serializable {
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "questionId", nullable = false)
    @JsonBackReference
    private Question question;

}

Then I wanted to update an exiting Question entry with a new set of Alternative list. For this I'm calling merge method of my JpaDao element with the new Question object:

@Repository
public class JpaQuestionDao implements QuestionDao {
    @PersistenceContext
    private EntityManager em;

    @Transactional
    public Question update(Question question) {
        return em.merge(question);
    }
}

However, this is in fact merging the two lists: the one already in database and the new provided one. I haven't had any problem with such method when I have non-list objects, that's why I kept using merge method.

Is there an alternative method to not merge but only update a list?

解决方案

Retrieve the question object first using EntityManager.find(). Question object will become managed as a result. Since alternativeList is lazily loaded, you have to invoke a method on the collections object to fetch it and make it managed as well. Any updates to the Question object and to the list will be automatically sent to the DB.

@Transactional
public Question update(Question question) {
    Question q = em.find(Question.class, question.getId());
    q.setAlternativeList(null);
    q.setAlternativeList(question.getAlternativeList());
    ...

    return q;
}

Alternatively, you can try using orphanRemoval=true on your @OneToMany collection

@OneToMany(fetch = FetchType.LAZY, mappedBy = "question", cascade = CascadeType.ALL, orphanRemoval=true)
@JsonManagedReference
private List<Alternative> alternativeList;

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