JPA选择查询以使用@ManyToOne映射返回实体

Question

I am a beginner, learning JPA, for practice I was working on this problem where I have two entity classes Person and Gym.

Person has : - id (auto-generated) - name - age - Gym (Many to One mapping)

Gym has : - id (auto-generated) - name - rating - fee - List of Person (One to Many mapping)

Now, I have my PersonRepository which extends JpaRepository and have this following JPQL query where I am try to retrieve all persons with age < (some user input value)

The problem is the retrieved person list is always empty. I tried used fetch join but still it returns empty list.

What should be the appropriate JPQL query for this scenario ?

Thanks ! Balasubramanyam

Gym Entity

@Entity
public class Gym {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int gym_id;

    @NotNull
    private String name;

    @NotNull
    private String city;

    @NotNull
    @Max(5)
    private double rating;

    @NotNull
    private double fee;

    @OneToMany(mappedBy="gym", 
                cascade= {CascadeType.MERGE, CascadeType.PERSIST,
                        CascadeType.REFRESH}, fetch=FetchType.EAGER)
    @JsonManagedReference
    private List<Person> personList;

    public Gym() {
        super();
    }

    public Gym(int gym_id, @NotNull String name, @NotNull double rating, @NotNull double fee, List<Person> personList,
            @NotNull String city) {
        super();
        this.gym_id = gym_id;
        this.name = name;
        this.rating = rating;
        this.fee = fee;
        this.personList = personList;
        this.city = city;
    }
// getters and setters

Person Entity

@Entity
public class Person {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    @NotNull
    private String name;

    @NotNull
    private int age;

    @ManyToOne (cascade={CascadeType.MERGE, CascadeType.PERSIST,
        CascadeType.REFRESH, CascadeType.DETACH})
    @JoinColumn
    @JsonBackReference
    private Gym gym;

    public Person() {
        super();
    }

    public Person(int id, @NotNull String name, @NotNull int age, Gym gym) {
        super();
        this.id = id;
        this.name = name;
        this.age = age;
        this.gym = gym;
    }
// getters and setters

PersonRepository

public interface PersonRepository extends JpaRepository<Person, Integer>{

@Query("select p from Person p join fetch p.gym where p.age<=(:age)")
List<Person> filterByAge(@Param("age") int age);
}

In my service class this is what I am doing

List<Person> filteredPersonList = personRepository.filterByAge(age);
System.out.println(filteredPersonList); // prints empty

Answer 1

If you change your repository to this you wont need to construct the query and will work.

public interface PersonRepository extends JpaRepository<Person, Integer>{

     List<Person> findByAgeLessThanEqual(int age);

}