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:

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
}

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
  }

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
}

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
  }

Spring JPA Cardinality

Database relation using JPA Cardinality

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