删除OneToMany关系中的实体 [英] remove an entity in OneToMany relationship

问题描述

我很难理解在从OneToMany关系中移除一个实体时需要付出的最小努力。我发现很多例子只是将实体添加到这些集合中（并且完全正确），但删除实体更加困难。

I have trouble understanding what minimal effort is needed on my part in removing an Entity from a OneToMany relationship. I find a lot of examples just adding Entities to these sets (and that works out allright), but removing entities is much more difficult to find.

我有以下类：

I have the following class:

@Entity
public class Product {
    ... 
    OneToMany(mappedBy="product", orphanRemoval=true,
              cascade={CascadeType.DETACH,CascadeType.MERGE,CascadeType.PERSIST,CascadeType.REFRESH},fetch=FetchType.EAGER)
    Set<Expert> experts = new HashSet<Expert>();
    ...
}

@Entity
public class Expert {
    ...
    @ManyToOne(optional=false)
    Product product;

    @ManyToOne(optional=false)
    Person person;

    ...
}

（Person类似于产品）

(Person is similar to Product)

我拥有充满产品和专家的产品。我想从产品列表中删除专家，以便专家实体完全删除。我预计下面的代码就足够了：

I have it filled with Products and experts. I want to remove an expert from the list of products so the expert entity is removed completely. I would expect that either the following code would be enough:

Product aProduct = findAProduct(...);
Expert anExpert aProduct.getExperts.get(...); // Just get the first expert that I want removed
EntityManager em = entityManager();

  em.getTransaction().begin();
  aProduct.getExperts().remove(anExpert);
  em.merge(aProduct);
  em.getTransaction().commit();

或：

or:

  em.getTransaction().begin();
  em.remove(anExpert);
  em.getTransaction().commit();

这是否过于简单？ JPA做了什么，我该做什么？我之前通过使用查询解决了这个问题，但我希望JPA能为我做到这一点。

Is this too simplistic? What is done by JPA and what do I have to do myself? I have solved it before by just using queries but I expect that JPA can do this for me.

推荐答案

EntityManager em = entityManager();
// Retrieve the expert in the new EntityManager context 
Expert expertToDelete = em.find(Expert.class, anExpert.id);

em.getTransaction().begin();
expertToDelete.getProduct().getExperts().remove(expertToDelete);
expertToDelete.getPerson().getExperts().remove(expertToDelete);
em.remove(expertToDelete);
em.getTransaction().commit();

它可能很好地代替了通过find找回专家，您可以使用merge ）而不是上面的，但上面的工作似乎有效。

It could very well be that instead of retrieving the expert via the find, you can use a merge() on anExpert instead, but the above seems to work.

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