在JPA中删除子级时，保持实体关系同步 [英] Keeping entity relationship in sync when deleting child in JPA

问题描述

我了解到，您需要保持具有关联关系的实体同步，即，当您从父级中删除子级时，还应该将在子实体中将父级保留为null的属性.在我的示例中，我具有以下父实体:

I've read that you need to keep entities that have a relationship in sync, i.e. when you remove a child from a parent you should also set the attribute that holds the parent to null in the child entity. In my example I have the following parent entity:

public class Parent {
    @OneToMany(mappedBy = "parent", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Child> children;
}

还有孩子:

public class Child {
    @ManyToOne(optional = false)
    private Parent parent;

    public void setParent(Parent parent) {
        this.parent = parent;
    }
}

从父级删除子级的代码如下(在此示例中，Parent可以在其列表中多次具有相同的Child):

The code for removing a child from the parent is as follows (in this example a Parent can have the same Child in its list multiple times):

public void removeChild(Child child) {
    List<Child> childrenToRemove = this.children.stream()
        .filter(c -> c.equals(child))
        .collect(Collectors.toList());
    childrenToRemove.forEach(child -> child.setParent(null));
    this.children.removeAll(childrenToRemove);
}

我首先在每个子级上将Parent设置为NULL，然后将它们从集合中删除.这样可以使实体保持同步.我还可以做的是将removeChild代码更改为以下内容:

I first set the Parent to NULL on each of the children and then remove them from the collection. This keeps the entities in sync. What I could also do is change the removeChild code to the following:

public void removeChild(Child child) {
    this.children.removeIf(c -> c.equals(child));
}

当然，在这种情况下，这些实体不会保持同步，因为这些Child实体中的每个实体仍然具有对Parent的引用.为了弥补这一点，我可以在Child实体中添加以下内容:

Of course in this case the entities are not kept in sync because each of those Child entities still has a reference to the Parent. To remedy that I could add the following to the Child entity:

@PreRemove
public void preRemove() {
    this.parent = null;
}

我现在的问题是，如果Child实体也保存在其他父实体的列表中，例如实体AnotherParent也会保留Child实体的列表，那么我还应该在上述的@PreRemove方法中添加this.anotherParent = null吗?如果Child与其他实体具有单向关系(即另一侧没有保存Child实体的列表，应该将它们设置为null)怎么办?

My question now is, what if Child entity is also kept in a list of a different parent entity, e.g. the entity AnotherParent which also keeps a list of Child entities, should I then also add this.anotherParent = null to the @PreRemove method defined above? What if Child has unidirectional relationship with other entities (i.e. the other side doesn't keep a list of the Child entities, should they be set to null?).

推荐答案

如

As explained in this article, you should keep the bidirectional associations in sync so that the entity state transitions can propagate and to avoid hard-to-track bugs in your code.

我现在的问题是，如果子实体也保留在列表的列表中，该怎么办? 不同的父实体，例如实体AnotherParent 保留子实体的列表，然后我还应该添加 上面定义的@PreRemove方法的this.anotherParent = null吗?

My question now is, what if Child entity is also kept in a list of a different parent entity, e.g. the entity AnotherParent which also keeps a list of Child entities, should I then also add this.anotherParent = null to the @PreRemove method defined above?

如果当前正在运行的Persistence cOntext中未加载AnotherParent实体，则不必这样做，因为内存中不存在父级集合.

If the AnotherParent entity is not loaded in the currently running Persistence cOntext, you don't have to do that because the parent-side collection does not exist in memory.

如果Child与其他实体有单向关系怎么办 (即，另一方未保留子实体的列表， 它们设置为null吗?).

What if Child has unidirectional relationship with other entities (i.e. the other side doesn't keep a list of the Child entities, should they be set to null?).

如果不这样做，则会得到ConstraintViolationException，因为单向关联更像是多对多而不是一对多.有关更多详细信息，请查看我的"JPA关系"视频课程.

If you don't do that, you'll get a ConstraintViolationException because unidirectional associations are more like many-to-many than one-to-many. For more details, check out my JPA Relationships video course.

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