JPA 标准 API 加入

Question

Help me plz with one moment. I read about 10 articles already, but don't understand join moment. I have 2 tables:

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
}

and

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<>();
}

I want a query works like

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

And then get the result in jsp with

    <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>

And I just can't understand this JOIN with Criteria API. Can you help me ? Try to use this snippet, but get a error

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

Assuming you want to return an instance of News associated to an instance of NewsCategoryDict , your criteria query should look as follows:

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();
}

The modified lines are marked with #1 and #2 comments.

The query should return all News which have a matching NewsCategoryDict ; but News records which are not associated to a NewsCategoryDict record will not be returned.

Answer 2

Your error has nothing to do with the join! What is actually happening is that in the JSP fragment you are trying to access ${news.created} which does not exist in the NewsCategoryDict . I believe the error is in the JSP fragment, not in the Criteria query.

The way I understand it is that you want to list the News object, but in the query you are selecting the NewsCategoryDict and this is why at the end you end up with missing attribute because the NewsCategoryDict does not contain ${news.created}