Spring JPA Cardinality

Spring JPA Cardinality refers to the relationship between two entities regarding how many instances of one entity can be associated with a single instance of another entity. Here's a breakdown with code examples:

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• One-to-One (OneToOne): One entity instance can be associated with at most one instance of another entity, and vice versa.

@Entity
public class User {

  @Id
  private Long id;

  @OneToOne(mappedBy = "user")
  private Account account;

  // Getters and Setters
}

@Entity
public class Account {

  @Id
  private Long id;

  @OneToOne
  @JoinColumn(name = "user_id") // Optional for defining foreign key column
  private User user;

  // Getters and Setters
}

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• One-to-Many (OneToMany): One entity instance can be associated with many instances of another entity, but a single instance of the other entity can only be associated with one instance of the first entity.

    @Entity
    public class Instructor {
  
      @Id
      private Long id;
  
      @OneToMany(mappedBy = "instructor")
      private List<Course> courses;
  
      // Getters and Setters
    }
  
    @Entity
    public class Course {
  
      @Id
      private Long id;
  
      @ManyToOne
      @JoinColumn(name = "instructor_id") // Optional for defining foreign key column
      private Instructor instructor;
  
      // Getters and Setters
    }
  

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• Many-to-One (ManyToOne): Similar to One-to-Many, but reversed. Many entity instances can be associated with one instance of another entity.

@Entity
public class Order {

  @Id
  private Long id;

  @ManyToOne
  @JoinColumn(name = "customer_id") // Optional for defining foreign key column
  private Customer customer;

  // Getters and Setters
}

@Entity
public class Customer {

  @Id
  private Long id;

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

  // Getters and Setters
}

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• Many-to-Many (ManyToMany): Many entity instances can be associated with many instances of another entity. This usually involves a separate join table to manage the relationships.

    @Entity
    public class Student {
  
      @Id
      private Long id;
  
      @ManyToMany(mappedBy = "students")
      private List<Course> courses;
  
      // Getters and Setters
    }
  
    @Entity
    public class Course {
  
      @Id
      private Long id;
  
      @ManyToMany
      @JoinTable(name = "student_course",
          joinColumns = @JoinColumn(name = "course_id"),
          inverseJoinColumns = @JoinColumn(name = "student_id")) // Define join table details
      private List<Student> students;
  
      // Getters and Setters
    }