Spring Data - MongoRepository - @Query to find item within nested list of objects

Question

I'm trying to query MongoDB to return a single Answer object contained within a QuestionDocument object.

I am using Spring, MongoRepository, and JDK 11.

My QuestionDocument POJO:

@Data
@Document(collection = "Questions")
@AllArgsConstructor(onConstructor = @__(@Autowired))
@NoArgsConstructor
     public class QuestionDocument {
     @Id
     private String questionId;
     (...) 
     private List<Answer> answers;
     (...)
}

My Answer POJO:

@Data
public class Answer implements Serializable {
     private String answerId;
     (...)

My QuestionRepository:

@Repository
public interface QuestionRepository extends MongoRepository<QuestionDocument, String> {
     @Query(value = "{ { 'questionId' : ?0 }, { 'answers.$answerId' : ?1 } }")
     Answer findByQuestionIdAndAnswerId(String questionId, String answerId);

My QuestionServiceImpl:

public getAnswer(String questionId, String answerId){
     Answer answer = findByQuestionIdAndAnswerId(questionId, answerId);
     return answer;
}

protected Answer findByQuestionIdAndAnswerId(String questionId, String answerId){
     Answer answer;
     try {
     answer = questionRepository.findByQuestionIdAndAnswerId(questionId, answerId);
     } catch (Exception e) {
     throw new IllegalArgumentException("There is no answer with this ID.");
}
     return answer;
}

When I hit my endpoint in Postman, the correct response body appears, but all of its values are null. I have verified that the correct questionId and answerId are passed in my parameters.

I have also consulted several additional SO posts and Spring and MongoDB documentation, but so far, implementing what I've read regarding traversing nested objects by property hasn't helped.

How does my @Query value need to change to properly return a specific Answer object from this nested list of answers?

I have attempted to create findBy methods like: findByQuestion_Answers_AnswerId(String answerId);

I have attempted to add @DBRef above my List<Answer> answers, and adding @Document(collection = "Answers") and @Id above private String answerId; in my Answer POJO. I then cleared my database, created a new question and answer, and queried for the specific answerId, and still returned null data.

What I expect, is that given the questionId and answerId, the query will return one Answer object and its associated information (answerBody, answerAuthor, etc.).

My postman response states SUCCESS, but the data is null.

Answer 1

You can change the Query to this.

@Query(value = "{{'questionId' : ?0, 'answers.answerId' : ?1}}")

or, just define this method.

findByQuestionIdAndAnswerId(String questionId, String answerId);

The return type will be of QuestionDocument, not Answer.

More details here.