java - JPA Criteria API 加入

Question

请帮我一下。我已经阅读了大约 10 篇文章，但不了解 join moment。我有 2 个表:

public class News implements Serializable {
@Id
@GeneratedValue (generator = "increment")
@GenericGenerator (name = "increment", strategy = "increment")
private int id;
@Column
private String name;
@Column
private Date created;
@Column
private String data;
@ManyToOne (cascade = CascadeType.ALL, fetch = FetchType.LAZY)
@JoinColumn (name = "category_id")
private NewsCategoryDict category;
// getters, setters
}

和

public class NewsCategoryDict implements Serializable {
@Id
@GeneratedValue (generator = "increment")
@GenericGenerator (name = "increment", strategy = "increment")
private int id;
@Column
private String name;
@OneToMany (mappedBy = "category", cascade = CascadeType.ALL)
private List<News> news = new ArrayList<>();
}

我想要一个像这样的查询

SELECT * FROM news, categorynews WHERE news.category_id = categorynews.id;

然后用jsp得到结果

    <div id="list_news">
        <c:forEach items="${news}" var="news">
            <h5>${news.id} : ${news.name} - ${news.created} ; ${news.data} (${news.category.name})</h5>
        </c:forEach>
    </div>

而且我无法理解这个 JOIN with Criteria API。你能帮助我吗 ？尝试使用此代码段，但出现错误

public List<News> getAll() {
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<News> cq = cb.createQuery(News.class);
    Root<News> rootFromNews = cq.from(News.class);
    Join<NewsCategoryDict, News> join = rootFromNews.join("category");
    cq.select(join);

    return em.createQuery(cq).getResultList();
}

PropertyNotFoundException: Property 'created' not found on type ru.r1k0.spring.model.NewsCategoryDict

Answer 1

假设您要返回与 NewsCategoryDict 实例关联的 News 实例，您的条件查询应如下所示:

public List<News> getAll() {
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<News> cq = cb.createQuery(News.class);
    Root<News> rootFromNews = cq.from(News.class);
    Join<News, NewsCategoryDict> join = rootFromNews.join("category"); // #1
    cq.select(rootFromNews);  // #2

    return em.createQuery(cq).getResultList();
}

修改的行用#1 和#2 注释标记。

查询应该返回所有具有匹配的 NewsCategoryDict 的 News；但是与 NewsCategoryDict 记录无关的 News 记录将不会被返回。