JPA / MySQL - 这个JPQL查询有什么问题？

Question

I have entity 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>();
...

Entity 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;
...

Then, in the PostDAOBean is following method:

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;
}

When I call this method in servlet I get next exception:

[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)
...

You can see that named query is: SELECT p FROM Post p ORDER BY SIZE(p.comments) DESC .

What is the cause of exception?

EDIT OK, I see that function SIZE(p.comments) can not be used in JPQL to be ordered by it.

What is the alternative way of getting same data with JPQL? Proper JPQL query?

EDIT Trying to implement this query with JPA Criteria API:

    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(???));
                ...???...
}

Need help with JPA Criteria API.

Answer 1

Well, compiler's barking: "Encountered "SIZE" at character 31, but expected: ["AVG", "COUNT", "KEY", "MAX", "MIN", "SUM", "VALUE", ]." while parsing JPQL...."

SIZE is a function that doesn't conform to the specification of the ORDER BY clause:

The ORDER BY clause allows the objects or values that are returned by the query to be ordered. The syntax of the ORDER BY clause is orderby_clause ::= ORDER BY orderby_item {, orderby_item}* orderby_item ::= state_field_path_expression [ASC | DESC] It is legal to specify DISTINCT with MAX or MIN, but it does not affect the result. When the ORDER BY clause is used in a query, each element of the SELECT clause of the query must be one of the following: an identification variable x, optionally denoted as OBJECT(x), a single_valued_association_path_expression, or a state_field_path_expression.

Furthermore:

The SIZE function returns an integer value, the number of elements of the collection. If the collection is empty, the SIZE function evaluates to zero.

---

Try this:

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());
        }

}