[Java] [JPA] @mapkey example

Course.java

import javax.persistence.*;

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public int courseCode;
    public String name;

    ...
}

ER Diagram.png

Semester.java

import javax.persistence.*;

@Entity
public class Semester {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public int id;
    public int year;
    public int ordinal;

    ...
}

Student.java

import javax.persistence.*;
import java.util.Map;

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public int studentId;
    public String firstName;
    public String lastName;

    // (Required either @OneToMany --or--- @ManyToMany)
    @ManyToMany(cascade = CascadeType.ALL) // if the map value is NOT a basic type.
    @JoinTable(name = "ENROLLMENTS",
            joinColumns = @JoinColumn(name = "STUDENT"),
            inverseJoinColumns = @JoinColumn(name = "SEMESTER"))
    @MapKeyJoinColumn(name = "COURSE")
    public Map<Course, Semester> enrollment;

    ...
}

key\value	@CollectionTable	uni-directional @OneToMany	bi-directional @OneToMany	@ManyToMany
@MapKeyColumn	link	link	link	link
@MapKeyTemporal	link	link	link	link
@MapKeyEnumerated	link	link	link	link
@MapKeyJoinColumn	link	link	link	link

• @MapKeyColumn if the map key is a basic type.
  If you don't specify the column name, the name of the property is «field»_KEY
• @MapKeyEnumerated/[@MapKeyTemporal](https://docs.oracle.com/javaee/7/api/javax/persistence/MapKeyTemporal.html @MapKeyTemporal) if the map key type is respectively an enum or a Date.
• @MapKeyJoinColumn/@MapKeyJoinColumns if the map key type is another entity.

---

• If the map key is NON-basic entity, the map key MUST be persisted (saved) before persist the holder.
• If the relationship is many-to-many and the map value is NON-basic entity, the map value also MUST be persisted before persist the holder.

---

• @ElementCollection is required if the map value is a basic type.
  Corresponding config annotation are @CollectionTable and @Column.
• if the map value is NON-basic type, it's required either @OneToMany or @ManyToMany.
  Corresponding config annotation is @JoinTable.