Hibernate JPA 雙向關係 Bidirectional Relationships 的意思

在Hibernate JPA 的Entity實體關係中，可分為 Bidirectional（雙向）或 Unidirectional（單向）。Bidirectional的意思如下。

Bidirectional Relationships（雙向關係）指兩個關聯實體都有參考到另一個實體的屬性，也就是兩個實體彼此"知道"彼此的存在，所以兩實體都可透過其有關聯設定的屬性物件來存取被關聯的實體。

Bidirectional雙向關係中一邊為owning side，另一邊為inverse side。

inverse side必須使用@OneToOne，@OneToMany或@ManyToMany的mappedBy屬性來指明owning side的參照外鍵的屬性名稱。所以設定Hibernate JPA實體的雙向關係時，要先了解哪一邊是owning side，哪一邊是inverse side。

在一對多/多對一(One To Many / Many To One)的雙向關係中，多邊(many side)總是為owning side。多邊不應設定mappedBy。

在一對一(One To One)的雙向關係中，owning side為持有外鍵屬性的那一邊。

在多對多(Many To Many)的雙向關係中，因為多對多關係會有一個中介資料表，所以任一邊都可以是owning side（只能設定一邊為owning side）。

例如兩個關聯實體分別為「作者」及「書」，假設關係為作者(Author)有多本書(Book)，為一對多(One To Many)的關係。若兩實體設為bidirectional雙向關係，則表示Author物件知道Book物件，Book物件也知道Author物件。

依照上面所述，Author為單邊(one side)，Book為多邊(many side)，
所以Author為inverse side，Book為owning side，

則程式中設定如下。

Author

@Entity
@Table(name="Author")
public class Author {
    
    @Id
    @Column(name="AuthorId")
    private Integer authorId;
    
    @Column(name="AuthorName")
    private String authorName;
    
    @OneToMany(mappedBy="author")
    private Set bookSet;

    // getter and setter ommitted...
}

Book

@Entity
@Table(name="Book")
public class Book {
    
    @Id
    @Column(name="BookId")
    private Integer bookId;
    
    @Column(name="BookName")
    private String bookName;
    
    @ManyToOne
    @JoinColumn(name="AuthorId") // 外鍵欄位名稱
    private Author author; // 外鍵的屬性

    // getter and setter ommitted...
}

上面範例中，兩個實體都有參考到另一個實體的關聯屬性，Author為bookSet；Book為author。

Author為inverse side，所以在關聯屬性bookSet使用@OneToMany的mappedBy指明owning side實體中參考外鍵的屬性名稱，所以設為@OneToMany(mappedBy="author")。

以下節錄自The Java EE 6 Tutorial - Direction in Entity Relationships

Bidirectional Relationships

In a bidirectional relationship, each entity has a relationship field or property that refers to the other entity. Through the relationship field or property, an entity class’s code can access its related object. If an entity has a related field, the entity is said to “know” about its related object. For example, if Order knows what LineItem instances it has and if LineItem knows what Order it belongs to, they have a bidirectional relationship.

Bidirectional relationships must follow these rules.

• The inverse side of a bidirectional relationship must refer to its owning side by using the mappedBy element of the @OneToOne, @OneToMany, or @ManyToMany annotation. The mappedBy element designates the property or field in the entity that is the owner of the relationship.
• The many side of many-to-one bidirectional relationships must not define the mappedBy element. The many side is always the owning side of the relationship.
• For one-to-one bidirectional relationships, the owning side corresponds to the side that contains the corresponding foreign key.
• For many-to-many bidirectional relationships, either side may be the owning side.

參考：

• Direction in Entity Relationships
• Hibernate JPA 雙向一對多/多對一設定 Bidirectional One To Many / Many To One
• Hibernate JPA Entity 生命週期 (JPA Entity Lifecycle)
• Annotation Type ManyToMany
• Annotation Type OneToMany
• Annotation Type OneToOne
• Spring Data JPA @OneToMany LazyInitializationException could not initialize proxy - no Session