Spring JPA创建问题

因此，主题可以是“自引用”实体，即可以具有可选的父主题和可选的子主题集合。

主题可以嵌套到任何级别：TopicA> TopicA_1> Topic_A_1_1等。

通过在Topic中编写一个递归函数，我们可以走到树上，并在层次结构中任何级别的Topic上获取该主题及其所有子主题的问题。

话题：

Entity@Table(name = "topics")public class Topic{    @Id    private Long id;    @oneToMany(mappedBy = "parent")    private Set<Topic> subTopics;    @ManyToOne    @JoinColumn(name = "parent_id")    private Topic parent;    @oneToMany(mappedBy = "topic")    private Set<Question> questions;    //questions for this exact topic    public Set<Question> getQuestions(){        return questions;    }    //questions for this topic and all its sub-topics    public Set<Question> getAllQuestions(){        return getAllQuestions(this);    }    //recursive function to walk the topic tree and get all questions for each sub-topic    private Set<Question> getAllQuestions(Topic topic){        Set<Question> questions = new HashSet<>(topic.getQuestions());        for(Topic subTopic : topic.getSubTopics()){ questions.addAll(getAllQuestions(subTopic));        )        return  questions;    }}

题：

@Entity@Table(name = "questions")public class Question {    @ManyToOne    @JoinColumn(name = "topic_id")    private Topic topic;}

因此，通过引用某个主题，我可以只获取其直接问题，也可以获取其所有问题以及所有子主题（及其所有子主题.....）的问题。

Topic topic = topicRepository.findOne(someId);//only questions directly linked to this topicSet<Question> questions = topic.getQuestions();//all questions linked to this topic and its sub-topics to *n* levels.Set<Question> questions = topic.getAllQuestions();