使用JPA註解使用Hibernate來形容@OneToMany關係，其中外鍵只在子表

Question

我有，我很高興與現有的數據模型：

public class Garden { 
    private String name; // "Oak Grove" 
    private List<Plant> plants; 
} 

public class Plant { 
    private String name; // "Cherry Tomato" 
} 

我想在Hibernate中映射此符合下列條件：

1. 的Plant類在Java中不維護其父Garden參考。這使得Java層，IMO中的事情變得更加困難。

2. PLANT表應該有一個GARDEN_ID列，它是GARDEN(ID)列的外鍵。

我的初始設置中，除了@OneToMany之前：

@Entity(name = "GARDEN") 
public class Garden { 
    @Id 
    @Column(name = "ID", nullable = false) 
    @GeneratedValue(strategy = GenerationType.AUTO) 
    private long id; 

    @Column(name = "NAME", nullable = false) 
    private String name; // "Oak Grove" 

    // Not yet mapped 
    private List<Plant> plants; 
} 

@Entity(name = "PLANT") 
public class Plant { 
    @Id 
    @Column(name = "ID", nullable = false) 
    @GeneratedValue(strategy = GenerationType.AUTO) 
    private long id; 

    @Column(name = "NAME", nullable = false) 
    private String name; // "Cherry Tomato" 
} 

我怎麼能以這樣的方式外鍵參考保持在Plant定義的List<Plant> plants;的@OneToMany註解？

如果我再補充：

@OneToMany(cascade = { CascadeType.ALL }) 
@JoinColumn(name = "GARDEN_ID") 
private List<Plant> plants; 

然後保存與植物花園未能像這樣：

Caused by: org.h2.jdbc.JdbcSQLException: NULL not allowed for column "GARDEN_ID"; SQL statement: 
insert into PLANT (NAME, ID) values (?, ?) [23502-191] 

如此看來休眠是不是要堅持外鍵。有沒有辦法做到這一點，而不是完全扼殺我的對象模型？

編輯：我測試這一問題的方法是：

Garden garden = new Garden("Oak Grove"); 
garden.addPlant(new Plant("Cherry Tomato")); 
gardenManager.save(garden); 

其中save()方法看起來非常休眠安永：

public void save(T item) { 
    try (Session session = factory.openSession()) { 
     Transaction transaction = session.beginTransaction(); 
     try { 
      session.saveOrUpdate(item); 
      transaction.commit(); 
     } catch (Exception ex) { 
      System.out.println("Error occurred saving item: " + ex.getMessage()); 
      ex.printStackTrace(); 
      transaction.rollback(); 
     } 
    } 
} 

Answer 1

Yogesh Sakurikar接近，但雙向@JoinColumn是有點關閉。下面您將看到如何加入雙向或單向

@Entity(name = "GARDEN") 
public class Garden { 
    @Id 
    @Column(name = "ID", nullable = false) 
    @GeneratedValue(strategy = GenerationType.AUTO) 
    private long id; 

    @Column(name = "NAME", nullable = false) 
    private String name; // "Oak Grove" 

    // use this if you don't want a bi-directional relationship 
    // @OneToMany 
    // @JoinColumn(name = "ID", referencedColumnName="GARDEN_ID") 
    // private List<Plant> plants; 

    // use this if you want it bi-directional 
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "garden") 
    private Set<Plant> plants; 
} 

@Entity(name = "PLANT") 
public class Plant { 
    @Id 
    @Column(name = "ID", nullable = false) 
    @GeneratedValue(strategy = GenerationType.AUTO) 
    private long id; 

    @Column(name = "NAME", nullable = false) 
    private String name; // "Cherry Tomato" 

    // use this if you don't want a bi-directional relationship 
    // @Column(name="GARDEN_ID") 
    // private long gardenId; 

    // use this if you want a bi-directional relationship 
    @ManyToOne 
    @JoinColumn(name = "GARDEN_ID", referencedColumnName="ID", nullable = false) 
    private Garden garden; 

} 

下面的代碼假設爲雙向關係。否則，你需要知道你的Garden.id之前，你可以完整地描述任何一個孩子Plant

Garden garden = new Garden("Oak Grove"); 
Plant plant = new Plant("Cherry Tomato") 
plant.setGarden(garden); //don't forget to set the parent on the child 
garden.addPlant(plant); 
gardenManager.save(garden); 

Answer 2

對一對多的關係，如果該工廠將保持這種關係，您需要使用雙向定義它。 這裏是我認爲你應該能夠實現它：

@Entity(name = "GARDEN") 
public class Garden { 
    @Id 
    @Column(name = "ID", nullable = false) 
    @GeneratedValue(strategy = GenerationType.AUTO) 
    private long id; 

    @Column(name = "NAME", nullable = false) 
    private String name; // "Oak Grove" 

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "garden") 
    private List<Plant> plants; 
} 

@Entity(name = "PLANT") 
public class Plant { 
    @Id 
    @Column(name = "ID", nullable = false) 
    @GeneratedValue(strategy = GenerationType.AUTO) 
    private long id; 

    @Column(name = "NAME", nullable = false) 
    private String name; // "Cherry Tomato" 

    @ManyToOne 
    @JoinColumn(name = "GARDEN_ID", nullable = false) 
    Garden garden; 
} 

這種雙向的做法將讓實體管理器知道存在着兩者之間的關係，因爲在一個方面，它是一對多的，爲對方它變成了多對一。希望這會有所幫助。