I’m working on a Spring Boot application and I’m facing an issue with handling nested objects within an entity.

I have an entity called TopicEntity, which has a field topicContent defined as a Map<Integer, String>. This field is used to store key-value pairs of topic content, where the key represents the order index and the value represents the content.

Now, I need to extend this functionality to support storing nested objects within topicContent. Each nested object contains a question, correct answer, and incorrect answers. Here’s an example of the desired JSON structure:

json structure

I’ve tried changing the mapping of topicContent to Map<Integer, Object>, but I encountered these errors:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘entityManagerFactory’

Caused by: org.hibernate.MappingException: collection element mapping has wrong number of columns: com.Project.LearningHub.Entity.TopicEntity.topicContent type: object

Here’s my current code:

package com.Project.LearningHub.Entity;

import java.util.HashMap;
import java.util.Map;

import javax.persistence.CollectionTable;
import javax.persistence.Column;
import javax.persistence.ElementCollection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.MapKeyColumn;
import javax.persistence.Table;

import com.fasterxml.jackson.annotation.JsonBackReference;

@Entity
@Table(name = "tblTopic")
public class TopicEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int topicId;

    @JsonBackReference
    @ManyToOne
    @JoinColumn(name = "lesson_id")
    private LessonEntity lesson;

    @Column(name = "lesson_id", insertable = false, updatable = false)
    private int lessonId;

    private String topicTitle;

    @ElementCollection
    @CollectionTable(name = "topic_content_mapping", joinColumns = @JoinColumn(name = "topic_id"))
    @MapKeyColumn(name = "order_index")
    @Column(name = "content")
    private Map<Integer, String> topicContent = new HashMap<>();

    public TopicEntity() {
        super();
    }

    public TopicEntity(int topicId, LessonEntity lesson, String topicTitle, Map<Integer, String> topicContent) {
        this.topicId = topicId;
        this.lesson = lesson;
        this.topicTitle = topicTitle;
        this.topicContent = topicContent;
    }

    public int getTopicId() {
        return topicId;
    }

    public void setTopicId(int topicId) {
        this.topicId = topicId;
    }

    public LessonEntity getLesson() {
        return lesson;
    }

    public void setLesson(LessonEntity lesson) {
        this.lesson = lesson;
    }

    public String getTopicTitle() {
        return topicTitle;
    }

    public void setTopicTitle(String topicTitle) {
        this.topicTitle = topicTitle;
    }

    public Map<Integer, String> getTopicContent() {
        return topicContent;
    }

    public void setTopicContent(Map<Integer, String> topicContent) {
        this.topicContent = topicContent;
    }

    public int getLessonId(){
        return this.lessonId;
    }
}