Spring Data MongoDb - Find All nested objects

Question

Let's say I've these 2 documents:

 @Document
 public class Exam {
   private int examId;
   private List<Question> questions;

and

 public class Question {
   private int questionId;
   private String question;

I need to write a 'findAll' that returns a List of ALL Questions (or ideally only 'question' string in the Question object) for a certain 'Exam' object (examId == n) using MongoRepository or some other way in Java using Spring Data MongoDb, how do I do that?

{
    "_id" : ObjectId("xyz"),
    "_class" : "com.xxx.Exam",
    "examId" : 1,
    "questions" : [ 
        {"questionId" : 1, "question" : "xyz" },
        {"questionId" : 2, "question" : "abc" }
    ]
}

Your modeling is questionable. With MongoDB, you usually don't use a join; instead, you embed your Questions into the Exam. Then just retrieve the exam. — chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-, Commented Aug 12, 2016 at 19:16
Sorry. Question is NOT another Document. Just an embedded object. — DilTeam
– DilTeam, Commented Aug 12, 2016 at 19:48
Possible duplicate of spring data - Mongodb - findBy Method for nested objects — helmy
– helmy, Commented Aug 12, 2016 at 20:34
No. This is not a duplicate of that. I had already read that one. Here I need to get ALL the nested objects. In that one, the author is updating a particular 'question'. — DilTeam
– DilTeam, Commented Aug 12, 2016 at 20:45

Andriy Simonov · Accepted Answer · 2016-08-12 22:36:33Z

0

There more than one way to do that, one of them could be like this:

MatchOperation match = match(new Criteria("examId").is(1));
UnwindOperation unwind = unwind("questions");
ProjectionOperation project = project().andExclude("_id").and("questions.question").as("question");
Aggregation aggregation = newAggregation(match, unwind, project);
AggregationResults<DBObject> result = mongoOperations.aggregate(aggregation, "exams", DBObject.class);

result.forEach(new Consumer<DBObject>() {
    @Override
    public void accept(DBObject t) {
        System.out.println(t.toString());
    }
});
// otuput
// { "question" : "xyz"}
// { "question" : "abc"}

The result is mapped to DBObject here, however you can define more appropriate class.

answered Aug 12, 2016 at 22:36

Andriy Simonov

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2 Comments

DilTeam Over a year ago

Where's project() coming from?

Andriy Simonov Over a year ago

project() is a stage in the aggregation pipeline, it is in the query to flatten the nested "questions.question" documents.

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