JPA with Hibernate - 合并性能问题

Question

我偶然发现了EntityManager 的merge 方法的问题。

我有这门课

@Entity
public class QuestionList {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String name;

    @ManyToOne
    private UserEntity user;

    @OneToMany(cascade = CascadeType.ALL)
    private List<QuestionUnit> units = new ArrayList<QuestionUnit>();

    public List<QuestionUnit> getUnits() {
        return units;
    }

    public void setUnits(List<QuestionUnit> units) {
        this.units = units;
    }

    public Long getId() {
        return id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void addQuestionUnit(QuestionUnit QuestionUnit) {
        this.units.add(QuestionUnit);
    }

    public void setUser(UserEntity user) {
        this.user = user;
    }

    public UserEntity getUser() {
        return this.user;
    }

}

现在，如果我只修改 units 中的一个对象，合并就可以了，但如果我删除或添加新对象到集合中，合并发生的方式是我得到 delete from QuestionList_QuestionUnit where QuestionList_id=? 然后每个一行一行添加insert into QuestionList_QuestionUnit (QuestionList_id, units_id) values (?, ?)

我想知道，是否可以告诉 JPA/Hibernate 只添加/删除与添加/删除的对象关联的行？还是合并只适用于集合的方式，在我的情况下，我需要编写自己的查询来更新数据库。

我问这个问题是因为这个集合可能非常庞大，并且仅仅删除所有行并再次插入几乎所有行似乎不是更新表格的好方法。

编辑：根据要求：

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class QuestionUnit {

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private long id;

    @OneToOne(cascade = CascadeType.ALL)
    private AnswerUnit correctAnswer;

    public QuestionUnit(AnswerUnit correctAnswer) {
        this.correctAnswer = correctAnswer;
    }

    public void setCorrectAnswer(AnswerUnit correctAnswer) {
        this.correctAnswer = correctAnswer;
    }

    public AnswerUnit getCorrectAnswer() {
        return this.correctAnswer;
    }

    public long getId() {
        return id;
    }

    public void setId(long id){
        this.id=id;
    }

    public abstract Object getQuestionContent();

    public abstract String getTitle();

}

这个层次结构中有 3 个具体的类，例如

@Question("openQuestion")
@Entity
public class OpenQuestion extends QuestionUnit {

    public void setQuestionContent(String questionContent) {
        this.questionContent = questionContent;
    }

    private String questionContent;

    public OpenQuestion() {
        this(null, new OpenQuestionAnswer());
    }

    public OpenQuestion(String questionContent, AnswerUnit correctAnswer) {
        super(correctAnswer);
        this.questionContent = questionContent;
    }

    public String getQuestionContent() {
        return this.questionContent;
    }

    public String getTitle() {
        return questionContent;
    }
}

@Transactional(propagation = Propagation.REQUIRED)
public void update(QuestionList questionList) {
    entityManager.merge(questionList);
}

此方法用于所有更新，因为列表处于分离状态。

Answer 1

它认为问题是由于使用 List 而不是 Set 引起的。

@OneToMany(cascade = CascadeType.ALL)
private List<QuestionUnit> units = new ArrayList<QuestionUnit>();

改成

@OneToMany(cascade = CascadeType.ALL)
private Set<QuestionUnit> units = new HashSet<QuestionUnit>();

Hibernate Many to Many Relations Set Or List?