了解 Spring Boot 中的 JPA 映射：一對一、一對多、多對一和多對多關係-java教程-PHP中文網

Understanding JPA Mappings in Spring Boot: One-to-One, One-to-Many, Many-to-One, and Many-to-Many Relationships

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;
}

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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;
}

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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;
}

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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;
}

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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;

}

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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;

}

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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;
}

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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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