Select one column using Spring Data JPA

问题

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();
}

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

回答1:

Concept is : In your entity class create a constructor with only required instant variables. And use that constructor in the repository method shown below.

Lets say you have a interface Repository like below

1. Repository implementation:

   public interface UserRepository<User> extends JpaRepository<User,String>
   {
       @Query(value = "select new com.org.User(usr.userId) from User usr where usr.name(:name))")
       List<User> findUserIdAlone(@Param("name") String user);
   }
   

2. In Controller

   @RestController
   public class UserController 
   {
       @Autowired
       private UserRepository<User> userRepository; 
   
       @Res
       public ResponseEntity<User> getUser(@PathVariable("usrname") String userName)
       {
           User resultUser = usrRepository.findUserIdAlone(userName);
           return ResponseEntity.ok(resultUser);
       }
   }
   
   public class User 
   {
   
       private String userId,userName;
   
       public User(String userId) 
       {
           this.userId=userId;
       }
       // setter and getters goes here
   }
   

回答2:

This Works for me.

public interface UserDataRepository extends JpaRepository<UserData, Long> {

    @Query(value = "SELECT emp_name FROM user_data", nativeQuery = true)
    public List<Object[]> findEmp_name();
}


System.out.println("data"+  userDataRepository.findEmp_name());

The above line gave me this result :

data[abhijeet, abhijeet1, abhijeet2, abhijeet3, abhijeet4, abhijeet5]

回答3:

If you want to only return a single column you should look at Projections and Excerpts which will allow you to filter specific columns and other things that are usefule.

回答4:

If you need list all of the users, try select userName from Users, if you need one user use "where" look at spring data JPA http://docs.spring.io/spring-data/jpa/docs/current/reference/html/ , try change CrudRepository to JpaRepository

来源：https://stackoverflow.com/questions/28912422/select-one-column-using-spring-data-jpa