在实体中拥有一个属性，该属性包含关系实体的子级总数

Question

我有一个 Question 实体和一个 Answer 实体。我希望问题有一个包含答案数量的属性。

我知道我可以在 Question 中定义关系，以便将所有答案与问题一起加载，但这不是很有效。我在一个单独的请求中加载了一个问题的所有答案。

我知道我也可以计算一个问题有多少个答案（使用存储库方法），但我希望将该逻辑与 Question 实体耦合，以便理想情况下完成答案计数和加载问题对象在 JPA 的同一个 SQL 查询中，否则请求必须执行 20 次 SELECT COUNT 查询才能获得每个问题的答案数，因为每次加载 20 个问题。

这可能是用 Spring JPA 实现的吗？

Answer 1

经过大量搜索和尝试，在@marnish 和@Augusto 的帮助下，我找到了解决问题的方法。

我需要从我问错问题的问题开始，因为我想知道如何将这些数据实施到我的实体中，而这种思维方式导致我在寻找解决方案时遇到很多麻烦，因为这仅限于我能够实现的目标。最好将我的数据库对象与我的 JSON 对象分开，因为一个用于持久化数据，另一个用于呈现给客户端，而第一种方式几乎不链接到数据库列。瞬态属性对我来说也不是解决方案。

意识到（并接受）这一点，下一个问题是让新的表示对象与页面包装器很好地配合使用，因为我的问题需要分页。这是我写的解决方案：

问题作为链接到数据库表和列的实体

@Entity
@Table(name = "vragen")
// BaseEntity just contains the ID property
public class Question extends BaseEntity {
    @Column(name = "vraag")
    private String question;

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

    @ManyToOne
    @JoinColumn(name = "gebruiker")
    private User user;

    @Column(name = "beantwoord", insertable = false)
    private boolean answered;

    @Column(name = "gepost_op", insertable = false)
    private Timestamp postedOn;

    @Column(name = "actief", insertable = false)
    @JsonIgnore
    private boolean active;

    @Column(name = "bekeken", insertable = false, updatable = false)
    private int viewed;

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
            name = "vragen_tags",
            joinColumns = @JoinColumn(name = "vraag_id", referencedColumnName = "id"),
            inverseJoinColumns = @JoinColumn(name = "tag_id", referencedColumnName = "id"))
    private Set<Tag> tags;

    @OneToMany(mappedBy = "question")
    // Thanks to @manish we can do it more efficiently instead of loading all the Answer objects
    @LazyCollection(LazyCollectionOption.EXTRA)
    @JsonManagedReference
    private Set<Answer> answers;


    public Question() {}

    // getters and setters
}

QuestionDTO 作为 JSON 表示对象

public class QuestionDTO {
    private Integer id;
    private String question;
    private String message;
    private User user;
    private boolean answered;
    private Timestamp postedOn;
    private int viewed;
    private Set<Tag> tags;
    private int totalAnswers;


    public QuestionDTO() {}

    // getters and setters
}

QuestionServiceImpl 中的方法：

public Page<QuestionDTO> getAllQuestions(int page, int size, String filter) {
    List<QuestionDTO> questions = new ArrayList<>();
    Page<Question> questionPage;

    switch (filter) {
        case "popular":
            questionPage = questionRepository.findAllByActiveTrueOrderByViewedDescPostedOnDesc(PageRequest.of(page, size));
            break;

        case "answered":
            questionPage = questionRepository.findAllByActiveTrueAndAnsweredTrueOrderByPostedOnDesc(PageRequest.of(page, size));
            break;

        default:
            questionPage = questionRepository.findAllByActiveTrueOrderByPostedOnDesc(PageRequest.of(page, size));
            break;
    }

    // Here the entity data is being converted to the presentation DTO. I'm sure this can be done cleaner with for example Mapstruct as objectmapper.
    for (Question q : questionPage) {
        QuestionDTO questionDTO = new QuestionDTO();

        questionDTO.setId(q.getId());
        questionDTO.setQuestion(q.getQuestion());
        questionDTO.setMessage(q.getMessage());
        questionDTO.setUser(q.getUser());
        questionDTO.setAnswered(q.isAnswered());
        questionDTO.setPostedOn(q.getPostedOn());
        questionDTO.setViewed(q.getViewed());
        questionDTO.setTags(q.getTags());
        questionDTO.setTotalAnswers(q.getAnswers().size());

        questions.add(questionDTO);
    }
    // We want to keep all the pagination data that the method returned, so we need to wrap the questionDTO in a Page wrapper and pass all the data from Page<Question>
    return new PageImpl<>(questions, questionPage.getPageable(), questionPage.getTotalElements());
}

回答实体：

@Entity
@Table(name = "antwoorden")
public class Answer extends BaseEntity {
    @Column(name = "bericht")
    private String message;

    @ManyToOne
    @JoinColumn(name = "gebruiker")
    private User user;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "vraag")
    @JsonBackReference
    private Question question;

    @Column(name = "gepost_op", insertable = false)
    private Timestamp postedOn;

    @Column(name = "actief", insertable = false)
    private boolean active;


    public Answer() {}

    // getters and setters
}