CriteriaQuery中的子查询

问题描述

我有抽象类 Entity 和这两个实体 Role 和 User ，它们是从Entity扩展的：

@Entity@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Entity {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    protected long id = 0; }
    

@Entity
public class Role extends Entity{
    @Column(name = "NAME",unique = true,nullable = false)
    private String name;
}

@Entity
public class User extends Entity{
    @Column(name = "ABBREVIATION",nullable = false)
    private String abbreviation;

    //The user can have several roles
    @ManyToMany
    @JoinTable(
        name = "USER_ROLE",joinColumns = @JoinColumn(name = "USER_ID"),inverseJoinColumns = @JoinColumn(name = "ROLE_ID"))
        private Set<Role> roles = new HashSet<>();
}

现在，当我单击某个角色时，我希望显示用户的缩写。 它的本机查询是：

query = em.createNativeQuery("SELECT ABBREVIATION FROM dbtest.user WHERE ID IN"
                        + " (SELECT USER_ID FROM dbtest.user_role WHERE ROLE_ID = " + role.getId() + ")");

我现在已经创建了一个条件查询，它总是给我一个空列表。

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Role> cq = cb.createquery(Role.class);
Root<User> root = cq.from(User.class);

Subquery subquery = cq.subquery(Role.class);
Root subfrom = subquery.from(Role.class);

subquery.select(subfrom.get(Role_.id));
subquery.where(cb.equal(subfrom.get(Role_.id),role.getId()));

cq.multiselect(root.get(User_.abbreviation));
cq.where(cb.equal(root.get(User_.id),subquery));

query = em.createquery(cq);
return query.getResultList();

这是怎么了？有人可以帮忙吗？

解决方法

首先，设置正确的类型：

CriteriaBuilder cb = em.getCriteriaBuilder();
//CriteriaQuery<Role> cq = cb.createQuery(Role.class);
//The type of the data returned in multiselect.
CriteriaQuery<String> cq = cb.createQuery(String.class);
Root<User> root = cq.from(User.class);
//Add to compare
Join<User,Role> joinRole = root.join(User_.roles,JoinType.Inner);

//The type of the data returned in select.
//Subquery subquery = cq.subquery(Role.class);
Subquery<Long> subquery = cq.subquery(Long.class);
//Root subfrom = subquery.from(Role.class);
//We set the data type in the Root
Root<Role> subfrom = subquery.from(Role.class);

subquery.select(subfrom.get(Role_.id));
subquery.where(cb.equal(subfrom.get(Role_.id),role.getId()));

cq.multiselect(root.get(User_.abbreviation));
//cq.where(cb.equal(root.get(User_.id),subquery));
//To use in the where subquery,call the getSelection () method
cq.where(cb.equal(root.get(User_.id),subquery.getSelection()));

query = em.createQuery(cq);
return query.getResultList();

此外，主查询所在的位置不是等于，请对此进行更改

cq.where(cb.equal(root.get(User_.id),subquery.getSelection()));

以此

cq.where(joinRole.get(Role_.id).in(subquery.getSelection()));

,

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Role> cq = cb.createQuery(Role.class);
Root<User> root = cq.from(User.class);

Subquery<Long> subquery = cq.subquery(Long.class);
Root<Role> subqueryRoot = subquery.from(Role.class);
Predicate subqueryPredicate = 
        cb.equal(subqueryRoot.get(Role_.id),role.getId());

//!!! I suspect here you have to select USER_ID,but Role entity does not contain such field        
subquery.select(subqueryRoot.get(Role_.id)) 
        .where(subqueryPredicate); 

Predicate queryPredicate = cb.in(root.get(User_.id)).value(subquery); 
cq.multiselect(root.get(User_.abbreviation)).where(queryPredicate);