预计不会使用 Hibernate 投影进行 N+1 次查询

Question

Just face the N+1 query problem with such Spring Data repository

public interface ToDoRepository extends CrudRepository<ToDo, Long> {

    @Query("select new com.package.repr.ToDoRepr(t) from ToDo t " +
            "where t.user.id = :userId")
    List<ToDoRepr> findToDosByUserId(@Param("userId") Long userId);
}

I see in the logs one such query

Hibernate: select todo0_.id as col_0_0_ from todos todo0_ where todo0_.user_id=? ]

And N such queries

Hibernate: select todo0_.id as id1_0_0_, todo0_.description as descript2_0_0_, todo0_.target_date as target_d3_0_0_, todo0_.user_id as user_id4_0_0_, user1_.id as id1_1_1_, user1_.password as password2_1_1_, user1_.username as username3_1_1_ from todos todo0_ left outer join users user1_ on todo0_.user_id=user1_.id where todo0_.id=?

ToDoRepr is a simple POJO. With constructor which accepts ToDo entity as a parameter.

Here are two JPA entities I use in this query

@Entity
@Table(name = "todos")
public class ToDo {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column
    private String description;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    @Column
    private LocalDate targetDate;

    // geters, setters, etc.
}

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

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(unique = true, nullable = false)
    private String username;

    @Column(nullable = false)
    private String password;

    @OneToMany(
            mappedBy = "user",
            cascade = CascadeType.ALL,
            orphanRemoval = true
    )
    private List<ToDo> todos;

    // geters, setters, etc.
}

UPD. It possible to resolve the problem by that query but why it's not working with constructor which accepts entity as a parameter?

public interface ToDoRepository extends CrudRepository<ToDo, Long> {

    @Query("select new com.package.repr.ToDoRepr(t.id, t.description, t.user.username, t.targetDate) " +
            "from ToDo t " +
            "where t.user.id = :userId")
    List<ToDoRepr> findToDosByUserId(@Param("userId") Long userId);
}

Answer 1

This is a very common question so I created the article Eliminate Spring Hibernate N+1 queries detailing the solutions

The best practice with Hibernate is to define all associations as Lazy to avoid fetching it when you don't need it. For more reasons, check Vlad Mihalcea's article https://vladmihalcea.com/eager-fetching-is-a-code-smell/

For fixing your issue, in your class ToDo, you should define the ManyToOne as Lazy :

@Entity
@Table(name = "todos")
public class ToDo {

    ...

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

    ...

    // geters, setters, etc.
}

If you need to access the user in your ToDoRepr, it will not be loaded by default so you need to add it to your query:

• JPQL, use JOIN FETCH :

public interface ToDoRepository extends CrudRepository<ToDo, Long> {

    @Query("select new com.package.repr.ToDoRepr(t) " +
            "from ToDo t " +
            "inner join fetch t.user " +
            "where t.user.id = :userId")
    List<ToDoRepr> findToDosByUserId(@Param("userId") Long userId);
}

• JPA, use EntityGraph :

public interface ToDoRepository extends CrudRepository<ToDo, Long> {

    @EntityGraph(attributePaths = {"user"})
    List<ToDoRepr> findToDosByUser_Id(Long userId);
}

Answer 2

I want to collect here some workaround about this my own question. There is a simple solution without explicit JPQL queries. Spring Data JPA can treat any POJO with proper getters and setters as a projection.

Just that works perfectly for me

public interface ToDoRepository extends CrudRepository<ToDo, Long> {

    List<ToDoRepr> findToDosByUser_Id(Long userId);
}