Find all embedded documents from manual reference in MongoDB

Question

I use MongoDB and Spring Boot in a project. I used manual reference to point out a collection, My structure is as follows:

Reel collection

Answer 1

Your JSON stored in database has wrong structure. Your Reel class expects list of Category, but in database you have stored as nested object.

You need to add this stage just after $lookup

{
  "$addFields": {
    "category": {
      "$map": {
        "input": "$category.videos",
        "in": {
          "videos": "$$this"
        }
      }
    }
  }
}

Java code

public List<Reel> findById(String _id) {

    Aggregation aggregation = Aggregation.newAggregation(
            Aggregation.match(Criteria.where("_id").is(_id)),
            Aggregation.lookup(mongoTemplate.getCollectionName(Video.class), "category.videos", "_id", "category.videos"),
            new AggregationOperation() {

                @Override
                public Document toDocument(AggregationOperationContext context) {
                    return new Document("$addFields",
                            new Document("category", new Document("$map", new Document("input", "$category.videos")
                                    .append("in", new Document("videos", "$$this")))));
                }
            })
        .withOptions(AggregationOptions.builder().allowDiskUse(Boolean.TRUE).build());

    LOG.debug(
            aggregation.toString().replaceAll("__collection__", mongoTemplate.getCollectionName(Reel.class)));

    return mongoTemplate.aggregate(aggregation, mongoTemplate.getCollectionName(Reel.class), Reel.class)
            .getMappedResults();

}

Recomendations

1. Do not hard-code collection name, use better mongoTemplate.getCollectionName method
2. Always log aggregation pipeline before performing, it helps debugging.
3. If your collection will grow in the future, use {allowDiskUse: true} MongoDb aggregation option.