了解 Spring Boot 中的 JPA 映射：一對一、一對多、多對一和多對多關係

使用 Spring Boot 和 JPA（Java Persistence API）建立應用程式時，管理實體之間的關係至關重要。了解如何有效地映射這些關係將幫助您準確地建模資料並執​​行有效的查詢。在本指南中，我們將探討 Spring Boot 支援的不同類型的 JPA 映射：一對一、一對多、多對一和多對多。將提供有關單向和雙向映射的範例。

一對一關係

單向一對一

在單向一對一關係中，一個實體引用另一個實體，而第二個實體不知道第一個實體。

範例：

// User.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToOne;

@Data
@Entity
public class User {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String username;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "fk_profile_id", referencedColumnName = "id")
    private Profile profile;

}

// Profile.java
import javax.persistence.Entity;
import javax.persistence.Id;

@Data
@Entity
public class Profile {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String bio;
}

雙向一對一

在雙向一對一關係中，兩個實體互相引用。

範例：

// User.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToOne;

@Data
@Entity
public class User {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String username;

    @OneToOne(mappedBy = "user", cascade = CascadeType.ALL)
    private Profile profile;

}

// Profile.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.JoinColumn;

@Data
@Entity
public class Profile {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String bio;

    @OneToOne
    @JoinColumn(name = "user_id")
    private User user;
}

一對多關係

單向一對多

在單向一對多關係中，一個實體引用另一個實體的集合。

範例：

// Author.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import java.util.List;

@Entity
@Data
public class Author {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String name;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "fk_book_id", referencedColumnName = "id")
    private List<Book> books;
}

// Book.java
import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
@Data
public class Book {
    @Id
    private Long id;
    private String title;
}

雙向一對多

在雙向一對多關係中，兩個實體互相引用。

範例：

// Author.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.CascadeType;
import javax.persistence.FetchType;
import java.util.List;

@Entity
@Data
public class Author {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private List<Book> books;

}

// Book.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
@Data
public class Book {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String title;

    @ManyToOne(name = "fk_author_id", referencedColumnName = "id")
    private Author author;
}

多對一關係

單向多對一

在單向多對一關係中，多個實體引用單一實體。

範例：

// Order.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
@Data
public class Order {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String description;

    @ManyToOne
    private Customer customer;

}

// Customer.java
import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
@Data
public class Customer {

    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String name;

}

雙向多對一

在雙向多對一關係中，被引用實體具有引用實體的集合。

範例：

// Order.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
@Data
public class Order {

    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String description;

    @ManyToOne
    private Customer customer;

}

// Customer.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import java.util.List;

@Entity
@Data
public class Customer {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "customer")
    private List<Order> orders;

}

多對多關係

單向多對多

在單向多對多關係中，每個實體都有另一個實體的集合，但該關係不由任何一方明確管理。

範例：

// Student.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import java.util.Set;

@Entity
@Data
public class Student {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String name;

    @ManyToMany
    private Set<Course> courses;

}

// Course.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import java.util.Set;

@Entity
@Data
public class Course {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String title;

    @ManyToMany
    private Set<Student> students;
}

雙向多對多

在雙向多對多關係中，兩個實體相互引用，並且關係是明確管理的。

範例：

// Student.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import java.util.Set;

@Entity
@Data
public class Student {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String name;

    @ManyToMany(mappedBy = "students")
    private Set<Course> courses;
}

// Course.java
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import java.util.Set;

@Entity
@Data
public class Course {
    @Id
    @GeneratedValue(stratergy = GenerationType.Identity)
    private Long id;
    private String title;

    @ManyToMany
    private Set<Student> students;
}

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