使用 Spring Data JPA 选择一列 [英] Select one column using Spring Data JPA

问题描述

有谁知道如何使用 Spring Data JPA 获取单个列?我在 Spring Boot 项目中创建了一个如下所示的存储库，但在访问 Restful URL 时总是得到 {"cause":null,"message":"PersistentEntity must not be null!"} 错误.

Does anyone have any idea how to get a single column using Spring Data JPA? I created a repository like below in my Spring Boot project, but always get the {"cause":null,"message":"PersistentEntity must not be null!"} error when accessing the Restful URL.

@RepositoryRestResource(collectionResourceRel = "users", path = "users")
public interface UsersRepository extends CrudRepository<Users, Integer> {

    @Query("SELECT u.userName  FROM Users u")
    public List<String> getUserName();
}

然后，如果我访问像 ../users/search/getUserName 这样的 Restful URL，我会收到错误消息:{"cause":null,"message":"PersistentEntity 不能为空！"}

Then if I access the Restful URL like ../users/search/getUserName, I get the error: {"cause":null,"message":"PersistentEntity must not be null!"}

推荐答案

创建投影界面

public interface UserNameOnly {
    String getUserName();
}

然后在您的存储库界面中返回该类型而不是用户类型

Then in your repository interface return that type instead of the user type

public interface UserRepository<User> extends JpaRepository<User,String> {
    List<UsernameOnly> findNamesByUserNameNotNull();
}

投影接口中的 get 方法必须匹配 JPA 存储库中定义类型的 get 方法，在本例中为 User.findBySomePropertyOnTheObjectThatIsNotNull"允许您根据某些条件获取实体列表(而不是 Iterable)，对于 findAll 而言，如果唯一标识符(或任何其他 NonNull 字段)不为空，则可以简单地获取实体列表.

The get method in the projection interface must match a get method of the defined type on the JPA repository, in this case User. The "findBySomePropertyOnTheObjectThatIsNotNull" allows you to get a List of the entities (as opposed to an Iterable) based on some criteria, which for a findAll can simply be if the unique identifier (or any other NonNull field) is not null.

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