如何使用条件查询创建具有多个关系属性的JPA投影?

我有以下场景,在User@Entity中有一个组@Entity的@manytomynet集。我想使用一个DTO类返回一个投影,该类包含一些用户属性和集合。

@Entity
public class User {
    private String email;
    private String password;

    @JoinTable(name = "users_groups", joinColumns = {
        @JoinColumn(name = "user_pk")}, inverseJoinColumns = {
        @JoinColumn(name = "group_pk")})
   @ManyToMany(fetch = FetchType.EAGER)
   private Set<Permission> permissions;
}

@Entity
public class Group {
    private String description;
}


public class DTO {
    private String email;
    private Set<Group> groups;

    public DTO(String email, Set<Group> groups) {        
        this.email = email;
        this.groups = groups;
    }
}

这是我尝试用jpa标准创建到dto类的投影的失败尝试。

public DTO findByEmail(String email) {
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<DTO> cq = cb.createQuery(DTO.class);
    Root<User> root = criteriaQuery.from(User.class);
    Join<User, Group> groups = root.join("groups");
    criteriaQuery.where(cb.equal(root.get("email"), email));
    cq.select(cb.construct(DTO.class, root.get("email"), root.get("groups")));

    return em.createQuery(cq).getSingleResult();
}

我得到以下错误:

• 原因:javax.persistence.persistence exception:org.hibernate.exception.sqlgrammareexception:无法提取ResultSet
• 原因:org.postgresql.util.PSQLException:错误:语法错误位于或靠近“.”