JPA / MySQL - 这个JPQL查询有什么问题？

Question

我有实体Post ：

@Entity
@Table(name = "post")
@NamedQueries({
    @NamedQuery(name = "getNewestPosts", query = "SELECT p FROM Post p ORDER BY p.date DESC"), // getting resultList ordered by date
    @NamedQuery(name = "getMostVisitedPosts", query = "SELECT p FROM Post p ORDER BY p.visitors DESC"), // ordered by most visited
    @NamedQuery(name = "getMostCommentedPosts", query = "SELECT p FROM Post p ORDER BY SIZE(p.comments) DESC")
})
public class Post implements Serializable {
    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "post_id", unique = true, nullable = false)
    private Integer id; 

    @Column(name = "post_title", length=300, unique = false, nullable = false)
    private String title;

    @Column(name = "post_date", unique = false, nullable = false)
    private Date date;

    @Column(name = "post_summary", length=1000, unique = false, nullable = true)
    private String summary;

    @Column(name = "post_content", length=50000, unique = false, nullable = false)
    private String content;

    @Column(name = "post_visitors", unique = false, nullable = false)
    private Integer visitors;

    @ManyToOne
    @JoinColumn (name = "user_id", referencedColumnName="user_id", nullable = false)
    private User user;

    @ManyToOne
    @JoinColumn (name = "category_id", referencedColumnName="category_id", nullable = false)
    private Category category;

    @OneToMany(cascade = { ALL }, fetch = EAGER, mappedBy = "post")
    private Set<Comment> comments = new HashSet<Comment>();
...

实体Comment ：

@Entity
@Table(name = "comment")
public class Comment implements Serializable {
    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "comment_id", unique = true, nullable = false)
    private Integer id; 

    @Column(name = "comment_title", length=300, unique = false, nullable = false)
    private String title;

    @Column(name = "comment_date", unique = false, nullable = false)
    private Date date;

    @Column(name = "comment_content", length=600, unique = false, nullable = false)
    private String content;

    @ManyToOne
    @JoinColumn (name = "user_id", referencedColumnName="user_id", nullable = false)
    private User user;

    @ManyToOne
    @JoinColumn (name = "post_id", referencedColumnName="post_id", nullable = false)
    private Post post;
...

然后，在PostDAOBean有以下方法：

public List<Post> getMostCommentedPosts(int page, int postsPerPage){

    Query q = em.createNamedQuery("getMostCommentedPosts");
    q.setFirstResult(page - 1);
    q.setMaxResults(postsPerPage);
    List<Post> resultList = (List<Post>) q.getResultList();

    if (resultList.isEmpty())
        return null;
    else
        return resultList;
}

当我在servlet中调用此方法时，我得到下一个异常：

[07.01.2014 09:06] Class name: class mbs2.blog.server.session.PostDAOBean, method name: public java.util.List mbs2.blog.server.session.PostDAOBean.getMostCommentedPosts(int,int) started
[07.01.2014 09:06] Class name: class mbs2.blog.server.session.PostDAOBean, method name: public java.util.List mbs2.blog.server.session.PostDAOBean.getMostCommentedPosts(int,int)<openjpa-2.2.0-r422266:1244990 nonfatal user error> org.apache.openjpa.persistence.ArgumentException: "Encountered "SIZE" at character 31, but expected: ["AVG", "COUNT", "KEY", "MAX", "MIN", "SUM", "VALUE", <IDENTIFIER>]." while parsing JPQL "SELECT p FROM Post p ORDER BY SIZE(p.comments) DESC". See nested stack trace for original parse error.
    at org.apache.openjpa.kernel.jpql.JPQLParser.parse(JPQLParser.java:51)
    at org.apache.openjpa.kernel.ExpressionStoreQuery.newCompilation(ExpressionStoreQuery.java:154)
    at org.apache.openjpa.kernel.QueryImpl.newCompilation(QueryImpl.java:672)
    at org.apache.openjpa.kernel.QueryImpl.compilationFromCache(QueryImpl.java:654)
    at org.apache.openjpa.kernel.QueryImpl.compileForCompilation(QueryImpl.java:620)
    at org.apache.openjpa.kernel.QueryImpl.compileForExecutor(QueryImpl.java:682)
    at org.apache.openjpa.kernel.QueryImpl.compile(QueryImpl.java:589)
    at org.apache.openjpa.persistence.EntityManagerImpl.createNamedQuery(EntityManagerImpl.java:1038)
    at org.apache.openjpa.persistence.EntityManagerImpl.createNamedQuery(EntityManagerImpl.java:102)
    at org.apache.openejb.persistence.JtaEntityManager.createNamedQuery(JtaEntityManager.java:274)
    at mbs2.blog.server.session.PostDAOBean.getMostCommentedPosts(PostDAOBean.java:38)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
...

您可以看到命名查询是： SELECT p FROM Post p ORDER BY SIZE(p.comments) DESC 。

异常的原因是什么？

编辑好了，我看到函数SIZE(p.comments)不能在JPQL中用它来订购。

使用JPQL获取相同数据的另一种方法是什么？ 适当的JPQL查询？

编辑尝试使用JPA Criteria API实现此查询：

    public List<Post> getMostCommentedPosts(int page, int postsPerPage){

        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<Post> cq = cb.createQuery(Post.class);
        Root<Post> p = cq.from(Post.class);
        cq.select(p);
        cq.orderBy(cb.desc(???));
                ...???...
}

需要帮助JPA Criteria API。

Answer 1

好吧，编译器咆哮：“遇到”SIZE“在31号，但预期：[”AVG“，”COUNT“，”KEY“，”MAX“，”MIN“，”SUM“，”VALUE“，]。” 解析JPQL时....“

SIZE是一个不符合ORDER BY子句规范的函数：

ORDER BY子句允许对查询返回的对象或值进行排序。 ORDER BY子句的语法是orderby_clause :: = ORDER BY orderby_item {，orderby_item} * orderby_item :: = state_field_path_expression [ASC | DESC]使用MAX或MIN指定DISTINCT是合法的，但它不会影响结果。 当在查询中使用ORDER BY子句时，查询的SELECT子句的每个元素必须是以下之一：标识变量x，可选地表示为OBJECT（x），single_valued_association_path_expression或state_field_path_expression。

此外：

SIZE函数返回一个整数值，即集合的元素数。 如果集合为空，则SIZE函数的计算结果为零。

---

尝试这个：

public List<Post> getMostCommentedPosts(int page, int postsPerPage){

        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<Post> cq = cb.createQuery(Post.class);
        Root<Post> p = cq.from(Post.class);
        cq.select(p).where(cb.isNotEmpty(p.get("comments")));
        List list = em.createQuery(cq).getResultList();
        for(Set each : list)
        {
            System.out.println(each.size());
        }

}