Hibernate通过外键获取实体列表，没有属性答案

【问题标题】：Hibernate getting a list of entities through a foreign key, without attributeHibernate通过外键获取实体列表，没有属性
【发布时间】：2020-03-10 06:05:54
【问题描述】：

我想通过连接它们的特定corporate_id 从任务表中检索任务实体列表。我似乎遇到的问题是它找不到corporate_id 列，因为Hibernate 查看没有corporate_id 属性的类。有什么办法可以解决这个问题吗？

我已经尝试创建一个corporate_id 属性，但这给了我一个NullPointerException。

我有一个CorporateEvent 类：

@Entity
@Table(name="corporate_event")
public class CorporateEvent extends Event {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_generator")
    @SequenceGenerator(name = "id_generator", sequenceName = "corporate_event_id_seq", allocationSize = 1)
    @Column(name = "corporate_id", updatable = false, nullable = false)
    private int id;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="corporateEvent")
    private List<Task> tasks;

还有一个Task 类：

@Entity
@Table(name="task")
public class Task {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_generator")
  @SequenceGenerator(name="id_generator", sequenceName = "task_task_id_seq", allocationSize=1)
  @Column(name="task_id", updatable = false, nullable = false)
  private int id;

  @ManyToOne
  @JoinColumn(name="corporate_id")
  private CorporateEvent corporateEvent;

  @Column(name="name")
  private String taskName;

  @Column(name="description")
  private String description;

这是我目前尝试过的 Hibernate 方法：

public List<Task> getTasksByCorporateEvent(CorporateEvent corporateEvent){
       session = ConnectRepository.factory.getCurrentSession();
       session.beginTransaction();

       CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();
       CriteriaQuery<Task> criteriaQuery = criteriaBuilder.createQuery(Task.class);
       Root<Task> root = criteriaQuery.from(Task.class);

      criteriaQuery.select(root).where(criteriaBuilder.equal(root.get("corporateEvent.corporateID"), corporateEvent.getId()));
      Query<Task> query = session.createQuery(criteriaQuery);
      List result = query.getResultList();

还有这个：

List<Task> result = session.createQuery("SELECT t FROM Task t WHERE t.corporate_id = :id").setParameter("id", corporateEvent.getId()).getResultList();

这些是两个类的数据库表：

CorporateEvent表：

create table corporate_event
(
    title            varchar,
    date             varchar(1000),
    location         varchar,
    description      varchar,
    max_participants integer,
    expenses         varchar(2000),
    corporate_id     serial not null
        constraint corporate_event_pk
            primary key,
    tasks            varchar(4000)
);

alter table corporate_event
    owner to si3_2019_group_5;

create unique index corporate_event_id_uindex
    on corporate_event (corporate_id);

Task表：

create table task
(
    task_id      serial not null constraint task_pk primary key,
    name         varchar,
    description  varchar(1000),
    corporate_id integer
        constraint corporate_id
            references corporate_event
            on update cascade on delete cascade
);

alter table task owner to si3_2019_group_5;

create unique index task_task_id_index on task (task_id);

更新添加 Obi-Wan 所做的更正后，我无法再保存我的对象。这是堆栈跟踪：

------------------------------------------------------------------------
Building Eventer 1.0-SNAPSHOT
------------------------------------------------------------------------

--- maven-resources-plugin:2.5:resources (default-resources) @ Eventer ---
[debug] execute contextualize
Using 'UTF-8' encoding to copy filtered resources.
Copying 11 resources

--- maven-compiler-plugin:3.6.1:compile (default-compile) @ Eventer ---
Nothing to compile - all classes are up to date

--- exec-maven-plugin:1.2.1:exec (default-cli) @ Eventer ---
Nov 18, 2019 1:10:45 PM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate Core {5.4.8.Final}
Nov 18, 2019 1:10:45 PM org.hibernate.annotations.common.reflection.java.JavaReflectionManager <clinit>
INFO: HCANN000001: Hibernate Commons Annotations {5.1.0.Final}
Nov 18, 2019 1:10:46 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using Hibernate built-in connection pool (not for production use!)
Nov 18, 2019 1:10:46 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: using driver [org.postgresql.Driver] at URL [jdbc:postgresql://tek-mmmi-db0a.tek.c.sdu.dk:5432/si3_2019_group_5_db]
Nov 18, 2019 1:10:46 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {user=si3_2019_group_5, password=****}
Nov 18, 2019 1:10:46 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
Nov 18, 2019 1:10:46 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH000115: Hibernate connection pool size: 1 (min=1)
Nov 18, 2019 1:10:46 PM org.hibernate.dialect.Dialect <init>
INFO: HHH000400: Using dialect: org.hibernate.dialect.PostgreSQLDialect
Nov 18, 2019 1:10:50 PM org.hibernate.cfg.AnnotationBinder bindClass
WARN: HHH000503: A class should not be annotated with both @Inheritance and @MappedSuperclass. @Inheritance will be ignored for: com.mycompany.domain.event.Event.
Nov 18, 2019 1:10:50 PM org.hibernate.boot.internal.InFlightMetadataCollectorImpl addIdentifierGenerator
WARN: HHH000069: Duplicate generator name id_generator
Nov 18, 2019 1:10:50 PM org.hibernate.boot.internal.InFlightMetadataCollectorImpl addIdentifierGenerator
WARN: HHH000069: Duplicate generator name id_generator
Exception in thread "main" java.lang.NullPointerException
    at com.mycompany.repositories.EventRepository.saveCorporateEvent(EventRepository.java:57)
    at com.mycompany.domain.event.EventManager.createCorporateEvent(EventManager.java:30)
    at com.mycompany.eventer.Eventer.main(Eventer.java:64)
------------------------------------------------------------------------
BUILD FAILURE
------------------------------------------------------------------------
Total time: 7.987s
Finished at: Mon Nov 18 13:10:50 CET 2019
Final Memory: 9M/100M
------------------------------------------------------------------------
Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:exec (default-cli) on project Eventer: Command execution failed. Process exited with an error: 1 (Exit value: 1) -> [Help 1]

To see the full stack trace of the errors, re-run Maven with the -e switch.
Re-run Maven using the -X switch to enable full debug logging.

For more information about the errors and possible solutions, please read the following articles:
[Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

这是将corporateEvent 保存到数据库的方法：

public int saveCorporateEvent(CorporateEvent corporateEvent) {

        try {
            session = ConnectRepository.factory.getCurrentSession();
            session.beginTransaction();
            session.persist(corporateEvent);
            session.getTransaction().commit();
            return corporateEvent.getId();
        } catch (Exception e) {
            e.printStackTrace();

        } finally {
            session.close();
        }
        return 0;
    }

我还在 CorporateEvent 类的 OneToMany 注释中添加了 @OneToMany(cascade=CascadeType.ALL, mappedBy="corporateEvent")，因为它给了我很多其他异常（重复 ID 和“未保存的瞬态实例”）。

【问题讨论】：

标签： java postgresql hibernate maven

【解决方案1】：

您应该将CorporateEvent 中的One to Many 关系更新为

@OneToMany
@JoinColumn(name = "task_id")
private List<Task> tasks;

如果查询更改为，createQuery 应该可以工作

SELECT t FROM Task t WHERE t.corporateEvent.id = :id

但是这个 HQL 会在 Tasks 和 CorporateEvent 之间建立连接。为了移除连接，您可以在 Task 实体内创建一个新字段，以便仅加载 corporate_id 而不是获取整个 CorporateEvent 对象。

@Basic
@Column(name = "corporate_id")
private Integer corporateEventId

然后，您可以使用此 HQL

SELECT t FROM Task t WHERE t.corporateEventId = :id

【讨论】：

使用您的解决方案时，我在尝试保存对象时收到 nullPointerException。我不确定它是否是数据库，它没有正确映射。我只是更改了您的答案中所写的内容，其余内容与原帖相同。
能否将错误堆栈跟踪添加到问题描述中？
我现在添加了一个堆栈跟踪和保存方法作为问题的更新，我希望它有意义。