在这种情况下，answer_id是评级表中的外键，并且answer_id是answers表中的主键。我需要删除答案，但**“无法删除或更新父行：一个外键约束失败.**错误发生.如何设置此外键的ON DELETE SET NULL选项。

@Entity
@Table(name = "ratings")
public class Ratings {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long rating_id;

    @Column(nullable = false, unique = false, length = 45)
    private Short ratingValue;

    @ManyToOne(cascade = fetch = FetchType.LAZY)
    @JoinColumn(name = "userId")
    private User user;

    @ManyToOne(cascade = fetch = FetchType.LAZY)
    @JoinColumn(name = "answer_id")
    private Answer answer;

    @ManyToOne(cascade = fetch = FetchType.LAZY)
    @JoinColumn(name = "question_id")
    private Question question;
//getters and setters

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