休眠，JPA - 單向一對一

Question

我想用Hibernate和JPA實現一對一的關係。我有兩個類是層次結構的一部分：問題層次結構和答案層次結構。

@Entity 
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 
public abstract class QuestionUnit { 

    @Id 
    @GeneratedValue(strategy = GenerationType.TABLE) 
    private int id; 

    @OneToOne(cascade = CascadeType.ALL)  
    @PrimaryKeyJoinColumn 
    private AnswerUnit correctAnswer; 
...} 


@Entity 
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 
public abstract class AnswerUnit { 

    @Id 
    private int id; 

    public abstract Object getAnswerContent(); 

    public abstract boolean isEqual(AnswerUnit otherAnswer); 

    public int getId() { 
     return id; 
    } 
} 

我們有OpenQuestion和OpenAnswer作爲實現。

我想讓OpenQuetions的表具有自動生成的主鍵，並且具有OpenAnswer的表的主鍵與OpenQuestion表中的主鍵具有相同的值。

我試圖按照這裏的例子： http://docs.jboss.org/hibernate/annotations/3.5/reference/en/html/entity.html part 2.2.5.1。一比一。

但是，當我堅持了OpenQuestion，我得到的列id，questionContent和OpenQuestionAnswer與id，answerContent表OpenQuestion，但IDS的值不匹配。

那麼，我在哪裏犯了一個錯誤？

Answer 1

「OpenQuestion」中沒有「CorrectAnswer」列嗎？
這應該是「OpenAnswer」中的ID列的外鍵 -
即 - 這些應該是匹配的ID值。
如果您沒有此列 - 我懷疑您的JPA映射中有某種錯誤。
編輯 - 對我自己 -
請添加MappedSuperClass。請閱讀here。

Answer 2

我不知道你在做什麼，爲什麼你有一對一的關係從問題回答??邏輯告訴我一個問題可以有很多答案（可能只有一個正確的答案）。

，並使用該方法，你不應該需要關心這兩個ID的都是平等的，如果多數民衆贊成的情況下，你應該有一個這樣的模式：

@Entity 
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 
public abstract class QuestionUnit { 
    @Id ... private int id; 
    @OneToMany private List<AnswerUnit> answers; //optional assuming there's a list of answers for every question 
    @ManyToOne private AnswerUnit correctAnswer; 

    public void setCorrectAnswer(AnswerUnit answer){ 
     if(answer.getQuestion().equals(this)){ //or maybe instead answer.setQuestion(this) 
      this.correctAnswer = answer; 
     }else{ 
      throw new RuntimeError("Answer does not belong to this question"); 
     } 
    } 

    //Other getter and setters 
    //Override the "equals" method!!! 
} 


@Entity 
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 
public abstract class AnswerUnit { 
    @Id private int id; 
    @OneToMany private QuestionUnit question; 

    public abstract Object getAnswerContent();  
    public abstract boolean isEqual(AnswerUnit otherAnswer); //should't you just overwrite the equals method in here?  
    //Other getter, setters, etc 
} 

然後使用它，像這樣：

QuestionUnit q = new OpenQuestion(); 
List<AnswerUnit> answers = new List<OpenAnswer>(); 

AnswerUnit a1 = new OpenAnswer(); 
AnswerUnit a2 = new OpenAnswer(); 
AnswerUnit a3 = new OpenAnswer(); 

a1.setQuestion(q); 
a2.setQuestion(q); 
a3.setQuestion(q); 

//Also set the contents and other required info for the questions... 

//And then associate the instances: 
q.setAnswers(answers); 
q.setCorrectAnswer(a3); 

em.persist(q); 

這甚至會用開放和定期解答工作