Criteria API连接问题：不包含关系列的实体不能“连接”

Question

所需实体大致如下：

@Entity
@Table(name = "tb_users")
public class User {

    @Id
    @GeneratedValue
    private UUID userId;

    // Omitted other fields and getters/setters ...
}

@Entity
@Table(name = "tb_groups")
public class Group {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer groupId;

    // ....
}

@Entity
@Table(name = "tb_group_members")
public class GroupMember {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "group_id")
    private Group group;

    // getters & setters ...
}

这是与中间表的多对多关系：

集团

我想要实现的查询（通过Criteria API）基本上与以下JPQL相同：

select g from Group g join GroupMember gm on g = gm.group where gm.user = ? and  ...or whatever...

我尝试按如下方式构造CriteriaQuery：

User userToQuery = ...;
// ....
CriteriaQuery<Group> query = criteriaBuilder.createQuery(Group.class);
Root<Group> groupRoot = query.from(Group.class);
Root<GroupMember> gmRoot = query.from(GroupMember.class);
groupRoot.join(...).on(criteriaBuilder.equal(groupRoot, gmRoot.get("group"))); // I can't find a suitable join() method to specify the relationship
Predicate predicate = criteriaBuilder.equal(gmRoot.get("user"), userToQuery);
// ......

由于组实体没有关联关系，我不知道如何调用join（…）方法

我必须反向声明关联，即从Group成员实体声明关联：

CriteriaQuery<GroupMember> query = criteriaBuilder.createQuery(GroupMember.class);
Root<GroupMember> gmRoot = query.from(GroupMember.class);
gmRoot.join("group");
// ......

JPA的Criteria API是否有此限制？不包含关联字段的实体如何与其他实体连接？

Answer 1

基于Querydsl的实现：

private EntityManager em;

......

User user = ...; // User to query (may be null)
...
QGroup qGroup = QGroup.group;
QGroupMemeber qGMember = QGroupMember.groupMember;

JPAQueryFactory queryFactory = new JPAQueryFactory(em);
JPAQuery<Group> query = (user == null) ?
        queryFactory.selectFrom(qGroup) :
        queryFactory.select(qGroup).from(qGroup).join(qGMember).on(qGroup.eq(qGMember.group));

BooleanBuilder builder = new BooleanBuilder();
if (user != null) {
    builder.and(qGMember.user.eq(user));
}
// if (...) {
//     Other conditions that need to be added to the where clause
// }
query.where(builder);
// Paging condition or something
// ....

List<Group> result = query.fetch(); // search result