Spring Data MongoDB: Accessing and updating sub documents

Question

First experiments with Spring Data and MongoDB were great. Now I've got the following structure (simplified):

public class Letter {
  @Id
  private String id;
  private List<Section> sections;
}

public class Section {
  private String id;
  private String content;
}

Loading and saving entire Letter objects/documents works like a charm. (I use ObjectId to generate unique IDs for the Section.id field.)

Letter letter1 = mongoTemplate.findById(id, Letter.class)
mongoTemplate.insert(letter2);
mongoTemplate.save(letter3);

As documents are big (200K) and sometimes only sub-parts are needed by the application: Is there a possibility to query for a sub-document (section), modify and save it? I'd like to implement a method like

Section s = findLetterSection(letterId, sectionId);
s.setText("blubb");
replaceLetterSection(letterId, sectionId, s);

And of course methods like:

addLetterSection(letterId, s); // add after last section
insertLetterSection(letterId, sectionId, s); // insert before given section
deleteLetterSection(letterId, sectionId); // delete given section

I see that the last three methods are somewhat "strange", i.e. loading the entire document, modifying the collection and saving it again may be the better approach from an object-oriented point of view; but the first use case ("navigating" to a sub-document/sub-object and working in the scope of this object) seems natural.

I think MongoDB can update sub-documents, but can SpringData be used for object mapping? Thanks for any pointers.

Answer 1

I figured out the following approach for slicing and loading only one subobject. Does it seem ok? I am aware of problems with concurrent modifications.

Query query1 = Query.query(Criteria.where("_id").is(instance));
query1.fields().include("sections._id");
LetterInstance letter1 = mongoTemplate.findOne(query1, LetterInstance.class); 
LetterSection emptySection = letter1.findSectionById(sectionId);
int index = letter1.getSections().indexOf(emptySection);

Query query2 = Query.query(Criteria.where("_id").is(instance));
query2.fields().include("sections").slice("sections", index, 1);
LetterInstance letter2 = mongoTemplate.findOne(query2, LetterInstance.class);
LetterSection section = letter2.getSections().get(0);

This is an alternative solution loading all sections, but omitting the other (large) fields.

Query query = Query.query(Criteria.where("_id").is(instance));
query.fields().include("sections");
LetterInstance letter = mongoTemplate.findOne(query, LetterInstance.class); 
LetterSection section = letter.findSectionById(sectionId);

This is the code I use for storing only a single collection element:

MongoConverter converter = mongoTemplate.getConverter();
DBObject newSectionRec = (DBObject)converter.convertToMongoType(newSection);

Query query = Query.query(Criteria.where("_id").is(instance).and("sections._id").is(new ObjectId(newSection.getSectionId())));
Update update = new Update().set("sections.$", newSectionRec);
mongoTemplate.updateFirst(query, update, LetterInstance.class);

It is nice to see how Spring Data can be used with "partial results" from MongoDB.

Any comments highly appreciated!

Answer 2

I think Matthias Wuttke's answer is great, for anyone looking for a generic version of his answer see code below:

@Service
public class MongoUtils {

  @Autowired
  private MongoTemplate mongo;

  public <D, N extends Domain> N findNestedDocument(Class<D> docClass, String collectionName, UUID outerId, UUID innerId, 
    Function<D, List<N>> collectionGetter) {
    // get index of subdocument in array
    Query query = new Query(Criteria.where("_id").is(outerId).and(collectionName + "._id").is(innerId));
    query.fields().include(collectionName + "._id");
    D obj = mongo.findOne(query, docClass);
    if (obj == null) {
      return null;
    }
    List<UUID> itemIds = collectionGetter.apply(obj).stream().map(N::getId).collect(Collectors.toList());
    int index = itemIds.indexOf(innerId);
    if (index == -1) {
      return null;
    }

    // retrieve subdocument at index using slice operator
    Query query2 = new Query(Criteria.where("_id").is(outerId).and(collectionName + "._id").is(innerId));
    query2.fields().include(collectionName).slice(collectionName, index, 1);
    D obj2 = mongo.findOne(query2, docClass);
    if (obj2 == null) {
      return null;
    }
    return collectionGetter.apply(obj2).get(0);
  }

  public void removeNestedDocument(UUID outerId, UUID innerId, String collectionName, Class<?> outerClass) {
    Update update = new Update();
    update.pull(collectionName, new Query(Criteria.where("_id").is(innerId)));
    mongo.updateFirst(new Query(Criteria.where("_id").is(outerId)), update, outerClass);
  }
}

This could for example be called using

mongoUtils.findNestedDocument(Shop.class, "items", shopId, itemId, Shop::getItems);
mongoUtils.removeNestedDocument(shopId, itemId, "items", Shop.class);

The Domain interface looks like this:

public interface Domain {

  UUID getId();
}

Notice: If the nested document's constructor contains elements with primitive datatype, it is important for the nested document to have a default (empty) constructor, which may be protected, in order for the class to be instantiatable with null arguments.