Java 如何将一个集合中的mongo id作为外键添加到另一个集合中

在我的SpringBoot应用程序中，我有一组TODO和一组课程。在应用程序视图中，我返回课程集合并显示我需要的任何课程。TODO存储为1个列表，表示所有当前TODO。我想做的是返回每门课程的TODO列表。因此，当视图打开时，应用程序将显示课程以及该课程的个人待办事项列表

是否有一种方法可以使用现有代码合并新功能。我已经创建了前端逻辑，并希望保留它。我最初的想法是将课程id添加到

Todo.java

，但这不起作用

Todo.java

@Document(collection="todos")
public class Todo {
@Id
private String id;

@NotBlank
@Size(max=250)
@Indexed(unique=true)
private String title;

private Boolean completed = false;  

private Date createdAt = new Date();

public Todo() {
    super();
}

public Todo(String title) {
    this.title = title;
}   

public String getId() {
    return id;
}

public void setId(String id) {
    this.id = id;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public Boolean getCompleted() {
    return completed;
}

public void setCompleted(Boolean completed) {
    this.completed = completed;
}

public Date getCreatedAt() {
    return createdAt;
}

public void setCreatedAt(Date createdAt) {
    this.createdAt = createdAt;
}

@Override
public String toString() {
    return String.format(
            "Todo[id=%s, title='%s', completed='%s']",
            id, title, completed);
}       
}

@Repository
public interface TodoRepository extends MongoRepository<Todo, String> {
    public List<Todo> findAll();
    public Todo findOne(String id);
    public Todo save(Todo todo);
    public void delete(Todo todo);
}

TodoRepository.java

@Document(collection="todos")
public class Todo {
@Id
private String id;

@NotBlank
@Size(max=250)
@Indexed(unique=true)
private String title;

private Boolean completed = false;  

private Date createdAt = new Date();

public Todo() {
    super();
}

public Todo(String title) {
    this.title = title;
}   

public String getId() {
    return id;
}

public void setId(String id) {
    this.id = id;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public Boolean getCompleted() {
    return completed;
}

public void setCompleted(Boolean completed) {
    this.completed = completed;
}

public Date getCreatedAt() {
    return createdAt;
}

public void setCreatedAt(Date createdAt) {
    this.createdAt = createdAt;
}

@Override
public String toString() {
    return String.format(
            "Todo[id=%s, title='%s', completed='%s']",
            id, title, completed);
}       
}

@Repository
public interface TodoRepository extends MongoRepository<Todo, String> {
    public List<Todo> findAll();
    public Todo findOne(String id);
    public Todo save(Todo todo);
    public void delete(Todo todo);
}

---

将

courseid

添加到

todo

文档时遇到了什么问题？@Veeram我不确定如何将courseid添加到文档中，我遇到了语法错误。我是否应该将其添加为courseid的列表？使用dbref解决此问题