How to retrieve matching element in array in spring mongodb ?

Question

Im trying to retrieve a document with a specific '_id' and a single embedded document with another specific '_id'.

my document is represent a catalog and it contains an array of courses.

example data:

'_id': ObjectId('1111'),
'name': 'example catalog',
...
...
'courses': [
     { 
         '_id': ObjectId('2222'),
         'name': 'my course',
         ...
     },
     {
         ....
     }

In mongod I run this aggregation query, and get back what I wish for:

db.getCollection('catalogs').aggregate(
{ $match: { '_id': ObjectId('58e8da206ca4f710bab6ef74') } },
{ $unwind: '$courses' },
{ $match: { 'courses._id': ObjectId('58d65541495c851c1703c57f') } })

As I mentioned earlier, I've get back I single catalog instance with a single course instance within.

In my java repo, I was trying to do the same:

    Aggregation aggregation = Aggregation.newAggregation(
            Aggregation.match(Criteria.where(Catalog.ID_FIELD).is(catalogId)),
            Aggregation.unwind(Catalog.COURSES_FIELD, true),
            Aggregation.match(Criteria.where(Catalog.COURSES_FIELD + '.' + Course.ID_FIELD).is(embeddedCourseId))
    );
    AggregationResults<Catalog> results = mongoTemplate.aggregate(aggregation,
            Catalog.class, Catalog.class);

    List<Catalog> catalog  = results.getMappedResults();

But unfortunately, I've got an instance of my 'example catalog' with empty array of courses.

While debugging, I've found that inside results, there are two props that returns back. first one is what I've used, called mappedResults (represents the converted object returning from mongoDB) - contains an empty array of courses. the other one is the rawResults, (represents the data as DBObject) - contains the specific course I query for

my Catalog class contains an ArrayList (if that make any difference).

Please help and let me know what should I do to convert the results properly, or if I did something wrong in my code.

Solution

You can try below options. The key is to preserve the structure when mapping the response.

Regular Queries:

Using $positional projection

Query query = new Query();
query.addCriteria(Criteria.where("id").is(new ObjectId("58e8da206ca4f710bab6ef74")).and("courses.id").is(new ObjectId("58d65541495c851c1703c57f")));
query.fields().include("name").position("courses", 1);
List<Course> courses = mongoTemplate.find(query, Course.class);

Using $elemMatch projection

Query query = new Query();
query.addCriteria(Criteria.where("id").is(new ObjectId("58e8da206ca4f710bab6ef74")));
query.fields().include("name").elemMatch("courses", Criteria.where("_id").is(new ObjectId("58d65541495c851c1703c57f") ) );
List<Course> Course = mongoTemplate.find(query, Course.class);

Aggregation

Mongo Version >= 3.4 & Spring 1.5.2 Boot / Spring 1.10.1 Mongo.

You can use $addFields stage which will overwrite the courses field with the $filter value while keeping all the existing properties. I couldn't find any addFields builder in current spring version. So I have to use AggregationOperation to create a new one.

AggregationOperation addFields = new AggregationOperation() {
    @Override
    public DBObject toDBObject(AggregationOperationContext aggregationOperationContext) {
        DBObject dbObject =
                new BasicDBObject("courses",
                        new BasicDBObject("$filter",
                                new BasicDBObject("input", "$$courses").
                                        append("as", "course").
                                        append("cond",
                                            new BasicDBObject("$eq", Arrays.<Object>asList("$$course._id", new ObjectId("58d65541495c851c1703c57f"))))));
        return new BasicDBObject("$addFields", dbObject);
    }
};

Aggregation aggregation = Aggregation.newAggregation(
            Aggregation.match(Criteria.where("_id").is(new ObjectId("58e8da206ca4f710bab6ef74"))),
            addFields
 );

Mongo Version = 3.2 & Spring 1.5.2 Boot / Spring 1.10.1 Mongo..

The idea is still same as above but this pipeline uses $project so you'll have to add all the fields that you want to keep in final response. Also used spring helper methods to create the $filter pipeline.

Aggregation aggregation = newAggregation(
     Aggregation.match(Criteria.where("id").is(new ObjectId("58e8da206ca4f710bab6ef74"))),
     Aggregation.project("name")
                 .and(ArrayOperators.Filter.filter("courses").as("course")                          
                 .by(ComparisonOperators.Eq.valueOf("course._id").equalToValue(new ObjectId("58d65541495c851c1703c57f")))
                    ).as("courses")
 );

Mongo Version <= 2.6

You'll have to use $unwind and add a course field to have spring map it correctly.