Question

使用Spring数据将下面提供的实体存储到数据库时遇到问题。在这两者之间使用OneToOne关系。仅当parentObject显式设置为childObject时，它才有效。这不是一个很好的解决方案，因为我希望子对象根本没有父对象。有可能吗？相关的问题已发布here

@Entity
@Table(name = "parent_object")
public class ParentObject {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id", unique = true, nullable = false)
private Long id;

@Column(name = "age")
private Long age;

@OneToOne(cascade = CascadeType.ALL, mappedBy = "parentObject")
private ChildObject childObject;

public ParentObject() {
}

public ParentObject(Long age, ChildObject childObject) {
    this.age = age;
    this.childObject = childObject;
}

public Long getId() {
    return id;
}

public Long getAge() {
    return age;
}

public ChildObject getChildObject() {
    return childObject;
}

public void setChildObject(ChildObject childObject) {
    this.childObject = childObject;
}
}

@Entity
@Table(name = "child_object")
public class ChildObject {
@Id
@Column(name = "child_id", unique = true, nullable = false)
private Long id;

@MapsId
@OneToOne
@JoinColumn(name = "child_id")
private ParentObject parentObject;

@Column(name = "name")
private String name;

public ChildObject() {
}

public ChildObject(String name) {
    this.name = name;
}

public Long getId() {
    return id;
}

public ParentObject getParentObject() {
    return parentObject;
}

public String getName() {
    return name;
}

public void setParentObject(ParentObject parentObject) {
    this.parentObject = parentObject;
}
}

但是，我真的不喜欢将parentObject设置为ChildObject：

ChildObject childObject = new ChildObject("name");
ParentObject parentObject = new ParentObject(12L, childObject);
childObject.setParentObject(parentObject);

这是获得共享ID的唯一方法吗？如果未明确设置parentObject，则抛出以下错误：

Caused by: org.hibernate.id.IdentifierGenerationException: attempted to assign id from null one-to-one property [test.ChildObject.parentObject]
    at org.hibernate.id.ForeignGenerator.generate(ForeignGenerator.java:83) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:101) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.jpa.event.internal.core.JpaPersistEventListener.saveWithGeneratedId(JpaPersistEventListener.java:67) ~[hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]

Answer 1

由于您具有双向关系（两侧都有OneToOne注释），您需要在另一侧保持关系每一侧的引用，如果您不喜欢，则可以通过以下方式将关系转换为单向关系：从一侧删除引用和映射。

Answer 2

您可以轻松封装操作，以便不需要有问题的调用：实际上，您应该始终这样做，以便您的内存模型始终处于一致状态：

public class ParentObject{

    public ParentObject(Long age, ChildObject childObject) {
        this.age = age;
        this.childObject = childObject;

        childObject.setParentObject(this);
    }

    public void setChildObject(ChildObject childObject) {
        this.childObject = childObject;
        childObject.setParentObject(this);
    }
}

现在，您的Domain Model API的客户端不需要了解有关后向引用表单Child to Parent的任何信息。

ChildObject childObject = new ChildObject("name");
ParentObject parentObject = new ParentObject(12L, childObject); //all relationships now set correctly

OneToOne与共享id的关系要求孩子了解父母

2 个答案: