How to implement OneToOne entity without explicit id field

[Boris Romashov]

I want to have 2 tables, say A and B, that are in one-to-one relationship.
To do this in entity class for A I add the following:

@OneToOne
private B b;

This is still not valid in JPA because it requires an explicit id field. But I don't want redundant fields in database.
How can I solve this problem ?

[Cameron Wallace McKenzie]

Here's a scenario with an Exam and an ExamDetail:



As you can see, it's a one to one mapping. There does need to be a reference from one table to the other, that's for sure. That's not superfluous - that's necessary!

The code is below. The example comes from my book Hiberante Made Easy. You can find some more information about this example on my website.

 
@Entity
@Table(name = "exam", schema = "examscam")
public class Exam {
/* @Basic annotation has been
 added to all basic fields*/
 
  private int id;
  private String shortName;

  private ExamDetail detail;

  @Id
  @GeneratedValue
  @Column(name = "id")
  public int getId() { return id; }
  private void setId(int id) {this.id = id;}

 @OneToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
  @JoinColumn(name="detail_id")
  public ExamDetail getDetail() {return detail; }
  public void setDetail(ExamDetail detail) {
    this.detail = detail;
  }	

  @Basic
  public String getShortName() { return shortName;}
  public void setShortName(String shortName) {
    this.shortName = shortName;
  }
}

 @Entity
@Table(name = "exam_detail", schema = "examscam")
public class ExamDetail {
/* @Basic annotation has been 
added to all basic fields*/
  private int id;
  private String fullName;
  private int numberOfQuestions;
  private int passingPercentage;  
  private Exam exam;
  
  @Id
  @GeneratedValue
  @Column(name = "id")
  public int getId() {return id;}
  private void setId(int id) {this.id = id;}

  @OneToOne(cascade=CascadeType.ALL, mappedBy="detail")
  public Exam getExam(){return exam;}
  public void setExam(Exam exam){this.exam=exam;}

  @Basic
  public String getFullName() {return fullName;}
  public void setFullName(String fullName) {
    this.fullName = fullName;
  }

  @Basic
  public int getNumberOfQuestions() {
   return numberOfQuestions;
  }
 public void setNumberOfQuestions(int numberOfQuestions){
    this.numberOfQuestions = numberOfQuestions;
  }

  @Basic
  public int getPassingPercentage() {
   return passingPercentage;
  }
  public void setPassingPercentage(int passingPercentage){
    this.passingPercentage = passingPercentage;
  }
}

Regards!

-Cameron McKenzie

[Boris Romashov]

I repeat the following requirement:
>> I don't want redundant fields in database.

In your example the field details_id is redundant. One-to-one relationship can be organized using only id fields of entities Exam and ExamDetails. I want to know how to do this.

[Cameron Wallace McKenzie]

Boris Romashov wrote:I repeat the following requirement:
>> I don't want redundant fields in database.

No need to get testy. I'm really only trying to help.

I guess I've always been incorrectly taught that two associated table in a database needed to be referenced through a foreign key. It appears that this 'foreign key' idea that I've been misinformed about is superfluous and redundant.

How do you enforce a constrained association between two tables without an apparently redundant and superfluous foreign key? I'm curious.

Regards,

-Cameron McKenzie

[Boris Romashov]

Cameron Wallace McKenzie wrote:
How do you enforce a constrained association between two tables without an apparently redundant and superfluous foreign key?

Each field in the table can be used as foreign key, even primary key. In table A field id can be autoincrement, in table B it should be usual integer that is linked with table A.

The same happens when a table builds many-to-many relationship. It doesn't have any special primary keys. We use pair of foreign keys as a natural primary key. I want the same but for one foreign key instead of two.

Is it impossible in JPA ?