在我的Play应用程序中使用Ebean / JPA，如何删除OneToOne关系中的对象？ [英] Using Ebean/JPA in my Play application, how can I delete an object in a OneToOne relationship?

问题描述

我有以下课程：

import play.db.ebean.Model;
import javax.persistence.*;

@Entity
public class A extends Model {
    @Id
    private int id;

    /* Other irrelevant properties */

    @OneToOne(cascade = CascadeType.ALL, optional = true)
    private B b;
}

---

---

import play.db.ebean.Model;
import javax.persistence.*;

@Entity
public class B extends Model {
    @Id
    private int id;

    /* Other irrelevant properties */

    @OneToOne(mappedBy = "b")
    private A a;

    @OneToOne(cascade = CascadeType.ALL, optional = false)
    private C c;
}

---

---

import play.db.ebean.Model;
import javax.persistence.*;

@Entity
public class C extends Model {
    @Id
    private int id;

    /* Other irrelevant properties */

    @OneToOne(mappedBy = "c")
    private B B;
}

在数据库中，它看起来像这样：

In the database, it looks like this:

Table    a
Columns  id, ... (other irrelevant columns), b_id

Table    b
Columns  id, ...(other irrelevant columns), c_id

Table    c
Columns  id, ...(other irrelevant columns)

现在在一个控制器方法中（对于那些熟悉Play的人）我有一个A的对象，并希望从数据库中删除它的属性b 。这也应该级联到c，所以c也会被删除。
这是我目前的做法：

Now in one the controller methods (for those familiar with Play) I have an object of A, and would like to delete its property "b" from the database. This should also cascade to c, so c gets deleted too. This is my current approach:

B b = a.getB();
b.delete();

这会引发异常：

[PersistenceException：ERROR执行DML bindLog []错误[不能
删除或更新父行：外键约束失败
（ databasename 。 a ，CONSTRAINT fk_a_payme_4 FOREIGN KEY（ b_id ）
REFERENCES b （ id ））]]

[PersistenceException: ERROR executing DML bindLog[] error[Cannot delete or update a parent row: a foreign key constraint fails (databasename.a, CONSTRAINT fk_a_payme_4 FOREIGN KEY (b_id) REFERENCES b (id))]]

它基本上归结为我正在尝试删除b，而a仍然在b_id列中保存对b的外键引用，所以我应该首先设置引用null。

It basically boils down to the fact that I'm trying to delete b, while a still holds a foreign key reference to b in column b_id, so I should first set that reference to null.

在Java中，这转换为：

In Java, this translates to:

B b = a.getB();
a.setB(null);
a.update();
b.delete();

这会将对象a中b的引用设置为null并正确删除b对象，但是c不会从数据库中删除。 （为什么？我虽然cascade属性会照顾这个）
我发现修复此问题的唯一方法是明确删除c，所以这样：

This does set the reference to b in object a to null and correctly deletes the b object, but c does not get deleted from the database. (Why? I though the cascade property would take care of this) The only way I have found to fix this is to explicitly delete c as well, so like this:

B b = a.getB();
C c = b.getC();

b.setC(null);
b.update();
c.delete();

a.setB(null);
a.update();
b.delete();

我对此并不十分满意，因为已经有8行代码删除了两行在数据库中，如果这张照片中有更多的关系会更多。

I'm not very pleased about this though, because that's already 8 lines of code to delete two rows in the database, and it would be more if there were more relationships in this picture.

至于我的问题：

如何从a中删除b，从而首先自动删除a到b的引用，如何在删除b时确保删除c？

提前感谢您！

编辑：目前为止最好的想法：将ab关系的所有权移至b。

best idea so far: move the ownership of the a-b relationship to b.

推荐答案

尝试使用

class A{

   ......

   @JoinColumn(nullable=true)
   private B b;

   .......
}

参见有关详细信息，请转至此 @ManyToOne（可选= false）与@Column（nullable = false）之间的区别。

See this question for more info What is the difference between @ManyToOne(optional=false) vs. @Column(nullable=false).

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