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Java Springboot JpaRepository 可以动态加载关系数据吗

news 2025/7/23 7:12:55

问题

我遵循了几个教程，发现每次我向服务器发出 HTTP 请求时，JpaRepository 都会返回实体及其所有关系。

以下是代码

@Table(name = "provinces")
public class Province {@Id@GeneratedValue(strategy = GenerationType.IDENTITY)@Column(nullable = false)private Long id;private String name;@OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)@JoinColumn(name = "province_id")private List<City> cities;@OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)@JoinColumn(name = "province_id")private List<PreRelease> preReleases;@OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)@JoinColumn(name = "province_id")private List<Project> projects;
}

@Table(name = "cities")
public class City {@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "province_id")private Province province;@OneToMany(fetch = FetchType.LAZY)@JoinColumn(name = "city_id")private List<Project> projects;
}

public class Project {@Id@GeneratedValue(strategy = GenerationType.IDENTITY)@Column(nullable = false)private Integer id;@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "province_id")private Province province;@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "city_id")private City city;@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "pre_release_id")private PreRelease preRelease;
}

问题是：JpaRepository 能否动态加载关系数据，有时我确实想要它的关系，有时我不想要它的关系数据。

解答

首先，只有当您尝试访问相关实体时，才会加载它们，因为您fetch = FetchType.LAZY在关系中定义了

此外，这取决于你所说的关系。如果你的意思是只想加载相关实体的 id 而不是整个实体，我建议这样做（自 JAVA 17 起重要记录可用）：

@Query(SELECT new com.baeldung.jpa.records.ProjectRecord(p.id, p.province.id, p.city.id, p.preRelease.id) FROM Project p WHERE b.id = :id") 
ProjectRecord findProjectById(@Param("id") Long id);

在存储库中使用此注释仅选择 id 并将其写入单独的模型，因此它不会加载所有相关实体。如果这不起作用，也许尝试在 @Query 中使用 nativeQuery=true 并从 JPQL 查询切换到本机 SQL:) https://www.baeldung.com/spring-jpa-java-records

或者你也可以使用@JsonIgnore从数据库获取数据时要忽略的属性。

例如：

@Table(name = "cities")
public class City {@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "province_id")private Province province;@JsonIgnore                      //To ignore projects while fetching the data@OneToMany@JoinColumn(name = "city_id")private List<Project> projects;
}