如何在 JPA Query 中计算行表

Question

我是 Spring Boot 的新手。我有一个 mysql 数据库，我使用查询来计算表中的行数。但它不起作用，它仍然返回我的原始表数据。你能帮我检查一下我的代码吗？

这是我的实体：

@Entity
@Table(name = "act_id_membership", schema = "activiti", catalog = "")
@IdClass(ActIdMembershipEntityPK.class)
public class ActIdMembershipEntity {
    private String userId;
    private String groupId;

    @Id
    @Column(name = "USER_ID_")
    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    @Id
    @Column(name = "GROUP_ID_")
    public String getGroupId() {
        return groupId;
    }

    public void setGroupId(String groupId) {
        this.groupId = groupId;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        ActIdMembershipEntity that = (ActIdMembershipEntity) o;
        return Objects.equals(userId, that.userId) &&
                Objects.equals(groupId, that.groupId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(userId, groupId);
    }
}

这是我的查询：

@Repository
public interface MemershipRepository extends JpaRepository<ActIdMembershipEntity, String> {
    @Query ("select new com.example.activiti_restful.dtos.UserMembership(i.userId, count(i)) from ActIdMembershipEntity i where i.userId ='kermit'")
    UserMembership countMemberships(String userId);
}

更新代码： 我的服务等级：

@Service
public class MembershipService {
@Autowired
private MemershipRepository repository;

public long count() {
    return repository.count();
}

我的资源类：

@RestController
public class MembershipResource {
@Autowired
private MembershipService membershipService;

@GetMapping("/membership")
public long list() {return membershipService.count();}
}

我的自定义 JSON 对象类：

public class UserMembership {
    private String userId;

    private long numberOfusers;

    public UserMembership(String userId, long numberOfusers) {
        this.userId = userId;
        this.numberOfusers = numberOfusers;
    }
} 

MySQL 表： act_id_membership

Answer 1

根据repositories documentation 使用CrudRepository 提供了一个名为count() 的方法，它是实现JpaRepository 的Superinterfaces 之一。

基于CrudRepository documentation 说：

长计数();返回实体的数量。

那么你应该使用CrudRepository 方法。除了Remove Uppercase MembershipREPOSITORY, by java code convention，你必须按照如下方式使用MembershipRepository。

 @Repository
 public interface MembershipRepository extends JpaRepository <ActIdMembershipEntity, String> {
 }

并在您的服务中使用它：

 @Service
 public class MembershipService {

     @Autowired
     private MembershipRepository repo;

     public long count() {
         return repo.count();
     }
 }

更新

根据您的要求： 在控制器中：

@RestController
public class MembershipResource {

    @Autowired
    private MembershipService membershipService;

    @GetMapping("/membership")
    public List<Object> list() { return membershipService.countMemberships();
    }
}

服务中：

@Service
public class MembershipService {

    @Autowired
    private MemershipRepository repository;

    public List<Object> countMemberships() {
        return repository.countMemberships();
    }

}

在存储库中：

@Repository
public interface MemershipRepository extends JpaRepository<ActIdMembershipEntity, String> {
    @Query ("select i.userId, count(i) from ActIdMembershipEntity i where i.userId ='kermit'")
    List<Object> countMemberships();
}

Answer 2

*> 实际上我希望它返回一个像 [{ name: kermit, value:6}] 这样的 json 格式。现在它只返回一个数字 6。我怎么能这样做？谢谢！

首先，创建一个类来包装你的数据：

public class UserMembership {

 private String userId;

 private long numberOfUsers;

 public UserMembership(String userId, long numberOfUsers) {
       this.userId = userId;
       this.numerOfUsers = numberOfUsers;     
 }

}

然后

@Repository 
public interface MembershipRepository extends JpaRepository <ActIdMembershipEntity, String> {
     @Query ("select new *****.UserMembership(i.userId, count(i)) from ActIdMembershipEntity i where i.userId = :userId")
     UserMembership countMemberships(String userId);
}

*****：你的完整包名

希望对您有所帮助！