FetchMode如何在Spring Data JPA中工作

Question

我确实在项目中的三个模型对象之间有关系（文章末尾的模型和存储库片段）。

当我调用PlaceRepository.findById它时，会触发三个选择查询：

（“ sql”）

SELECT * FROM place p where id = arg

SELECT * FROM user u where u.id = place.user.id

SELECT * FROM city c LEFT OUTER JOIN state s on c.woj_id = s.id where c.id = place.city.id

（对我而言）这是非常不正常的行为。据阅读Hibernate文档后所知，它应该始终使用JOIN查询。在类中FetchType.LAZY更改为 查询时（带有附加SELECT 的查询）没有任何区别，而在类更改为 （使用JOIN查询时）则没有变化。FetchType.EAGERPlaceCityFetchType.LAZYFetchType.EAGER

当我使用CityRepository.findById抑制射击时，有两个选择：

SELECT * FROM city c where id = arg

SELECT * FROM state s where id = city.state.id

我的目标是在所有情况下都具有sam行为（始终为JOIN或SELECT，但首选JOIN）。

型号定义：

地点：

@Entity

@Table(name = "place")

public class Place extends Identified {

    @Fetch(FetchMode.JOIN)

    @ManyToOne(fetch = FetchType.LAZY)

    @JoinColumn(name = "id_user_author")

    private User author;

    @Fetch(FetchMode.JOIN)

    @ManyToOne(fetch = FetchType.LAZY)

    @JoinColumn(name = "area_city_id")

    private City city;

    //getters and setters

}

市：

@Entity

@Table(name = "area_city")

public class City extends Identified {

    @Fetch(FetchMode.JOIN)

    @ManyToOne(fetch = FetchType.LAZY)

    @JoinColumn(name = "area_woj_id")

    private State state;

    //getters and setters

}

仓库：

PlaceRepository

public interface PlaceRepository extends JpaRepository<Place, Long>, PlaceRepositoryCustom {

    Place findById(int id);

}

UserRepository：

public interface UserRepository extends JpaRepository<User, Long> {

        List<User> findAll();

    User findById(int id);

}

城市资料库：

public interface CityRepository extends JpaRepository<City, Long>, CityRepositoryCustom {    

    City findById(int id);

}

Answer 1

我认为Spring Data会忽略FetchMode。在使用Spring Data时，我总是使用@NamedEntityGraph和@EntityGraph批注@Entity@NamedEntityGraph(name = "GroupInfo.detail",&nbsp; attributeNodes = @NamedAttributeNode("members"))public class GroupInfo {&nbsp; // default fetch mode is lazy.&nbsp; @ManyToMany&nbsp; List<GroupMember> members = new ArrayList<GroupMember>();&nbsp; …}@Repositorypublic interface GroupRepository extends CrudRepository<GroupInfo, String> {&nbsp; @EntityGraph(value = "GroupInfo.detail", type = EntityGraphType.LOAD)&nbsp; GroupInfo getByGroupName(String name);}

Answer 2

首先，@Fetch(FetchMode.JOIN)并且@ManyToOne(fetch = FetchType.LAZY)是对立的，一个指示EAGER的获取，而另一个则指示LAZY的获取。急切的获取很少是一个好的选择，对于可预期的行为，最好使用query-time JOIN FETCH指令：public interface PlaceRepository extends JpaRepository<Place, Long>, PlaceRepositoryCustom {&nbsp; &nbsp; @Query(value = "SELECT p FROM Place p LEFT JOIN FETCH p.author LEFT JOIN FETCH p.city c LEFT JOIN FETCH c.state where p.id = :id")&nbsp; &nbsp; Place findById(@Param("id") int id);}public interface CityRepository extends JpaRepository<City, Long>, CityRepositoryCustom {&nbsp;&nbsp; &nbsp; @Query(value = "SELECT c FROM City c LEFT JOIN FETCH c.state where c.id = :id")&nbsp; &nbsp;&nbsp; &nbsp; City findById(@Param("id") int id);}