jpa_example.java

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

import static java.util.Collections.unmodifiableList;
import static javax.persistence.GenerationType.AUTO;

@Entity
public class Issue {

    @EmbeddedId
    private IssueId id;
    private String name;
    private IssueStatus status;
    @OneToMany(cascade = CascadeType.MERGE)
    private List<IssueComment> comments;
    ...
    public void changeStatusTo(IssueStatus newStatus) {
        if (this.status == IssueStatus.DONE && newStatus == IssueStatus.NEW || this.status == IssueStatus.NEW && newStatus == IssueStatus.DONE) {
            throw new RuntimeException(String.format("Cannot change issue status from %s to %s", this.status, newStatus));
        }
        this.status = newStatus;
    }
    ...
}