Spring boot JPA & Criteria API - Select single column

问题

I am trying to retrieve a single column from the table, but I am getting compilation error about return type.

SQL

select oComment from comment where oNote = note and version > 0;

I have Comment table and Note table. Comment table has comment, note and version columns. The comment itself is a note. Now I want to retrieve all comments of the note which has version greater than 0. But here I want only comment column which of note type.

Comment.java

    @Entity
    @Table(name="comment")
    @Cache(usage=CacheConcurrencyStrategy.READ_WRITE, region="comments")
    public class Comment implements Serializable {

        private static final long serialVersionUID = -4420192568334549165L;

        public Comment() {
        }

        @Id
        @OneToOne
        @JoinColumn(name="commentuuid",referencedColumnName="noteuuid")
        private Note oComment;

        @Id
        @OneToOne
        @JoinColumn(name="noteuuid",referencedColumnName="noteuuid")
        private Note oNote;
}

Note.java

@Entity
@Table(name = "note")
@Cache(usage=CacheConcurrencyStrategy.READ_WRITE, region="notes")
public class Note implements Serializable{

    private static final long serialVersionUID = 4089174391962234433L;

    @Column(name="title")
    private String m_szTitle;

    @Column(name="htmlcontent")
    private String m_sbHtmlContent;

    @Column(name="textcontent")
    private String m_sbTextContent;

    @Id
    @Column(name="noteuuid", columnDefinition="varchar(36)")
    private String noteUuid;
}

CustomRepositoryMethod

public List<Note> findAllByNote(Note oNote, int iOffset, int iResultSize) {

        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<Comment> cq = cb.createQuery(Comment.class);

        Root<Comment> oComment = cq.from(Comment.class);
        List<Predicate> predicates = new ArrayList<>();

        predicates.add(cb.equal(oComment.get("oNote"), oNote));
        predicates.add(cb.greaterThan(oComment.get("version"), 0));

        Subquery<Note> subquery = cq.subquery(Note.class);
        Root<Note> note = subquery.from(Note.class);

        cb.desc(note.get("m_dtCreationDate"));

        cq.where(predicates.toArray(new Predicate[0]));

        cq.multiselect(oComment.<Note>get("oComment"));

        return (List<Note>)em.createQuery(cq).setFirstResult(iOffset).setMaxResults(iResultSize).getResultList();
    }

error Error at return statement,

Cannot cast from List<Comment> to List<Note>

回答1:

in CustomRepositoryMethod replace first line CriteriaQuery<Comment> cq = cb.createQuery(Comment.class); to CriteriaQuery<Note> cq = cb.createQuery(Note.class)

cb.createQuery parameter accept result Class in docs you can see.

update

// assuming query like
// select oComment from comment inner join Note on comment.noteuuid=Note.noteuuid where Note.noteUuid = 1 and version > 0;

CriteriaBuilder cb = em.getCriteriaBuilder();
// data type of oComment
CriteriaQuery<Note> query = cb.createQuery(Note.class);
// from comment
Root<Comment> comment = query.from(Comment.class);

//join
Join<Comment, Note> note = comment.join(comment.get("oNote"));

//version Condition
Predicate version=cb.greaterThan(comment.get("version"),0 );

//Note condition
predicate note=cb.equal(note.get("noteuuid"),note.getNoteUuid());

// get oComment and where condtion 
query.select(comment.get("oComment")).where(cb.and(version,note));

    return  em.createQuery(query).setFirstResult(iOffset).setMaxResults(iResultSize).getResultList();

回答2:

Your criteria query's root is Comment not Note

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Comment> cq = cb.createQuery(Comment.class);
Root<Comment> oComment = cq.from(Comment.class);

and you are trying to do

return (List<Note>)em.createQuery(cq).setFirstResult(iOffset)
.setMaxResults(iResultSize).getResultList();

the compilation error is inevitable in this scenario, because em.createQuery(cq).getResultList() will return List<Comment> not List<Note>

回答3:

It is not necessary to write a custom repository method since the one you are creating is already generated in spring-data.

If your repository extends the CrudRepository you will be given that method you are looking for for free.

The pattern is findAllBy[propertyOfClass].

But please be aware of that you actually have no Collection of NOTE in your entity.

Perhaps you should first change the OneToOne association into a OneToMany.

回答4:

Can be built as a criteria query as follows:

CriteriaQuery<Country> q = cb.createQuery(Country.class);

Root<Country> c = q.from(Country.class);

q.select(c.get("currency")).distinct(true);

The select method takes one argument of type Selection and sets it as the SELECT clause content.

来源：https://stackoverflow.com/questions/56806095/spring-boot-jpa-criteria-api-select-single-column