How can JPA be used to map two composite foreign keys that refer to a single column?

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JPA allows mapping of composite foreign keys using the @JoinColumns annotation. To map two composite foreign keys that refer to a single column, we can use the following steps:

1. Define the entities for both the tables that have the composite foreign key association.

2. In the parent entity, specify the @OneToMany or @ManyToMany association for the child entity.

3. Use the @JoinColumns annotation to map the composite foreign key. In the @JoinColumns annotation, specify the name of the column and the names of the columns in the composite key of the parent table and the child table that refer to the same column.

For example, let's say we have two tables - employee and department. The employee table has a composite foreign key that refers to the id column of the department table. We can map this association using JPA as follows:

@Entity
public class Department {
   @Id
   private Long id;

   // other department related fields and methods
}

@Entity
public class Employee {
   @EmbeddedId
   private EmployeeId id;

   @ManyToOne
   @JoinColumns({
      @JoinColumn(name = "department_id", referencedColumnName = "id"),
      @JoinColumn(name = "region_id", referencedColumnName = "id")
   })
   private Department department;

   // other employee related fields and methods
}

@Embeddable
public class EmployeeId implements Serializable {
   // define the employee_id and region_id fields and methods
}

In the Employee entity, we specify the @ManyToOne association with the Department entity and use the @JoinColumns annotation to map the composite foreign key. The name attribute specifies the name of the column in the employee table that refers to the departmentid column in the department table. The referencedColumnName attribute specifies the name of the referenced column in the department table. The same applies to the regionid column.

With these annotations, we have successfully mapped two composite foreign keys that refer to a single column using JPA.