public class Parent {
    public static final String COLLECTION_NAMING = "Parent";
    ObjectId id;
    DBRef childDbRef;
    Child child;

    public Parent() {
    }

    public Parent(final ObjectId id) {
        this.id = Objects.requireNonNull(id);
    }

    public Parent(final ObjectId id, final Child child) {
        this.id = Objects.requireNonNull(id);
        this.child = Objects.requireNonNull(child);
        this.childDbRef = new DBRef(Child.COLLECTION_NAMING, child.id);
    }

    public ObjectId getId() {
        return id;
    }

    public void setId(ObjectId id) {
        this.id = id;
    }

    public Child getChild() {
        if (childDbRef != null && child == null) {
            throw new IllegalStateException("Not loaded");
        }
        return child;
    }

    public DBRef getChildDbRef() {
        return childDbRef;
    }

    public void setChild(final DBRef dbRef) {
        assert Child.COLLECTION_NAMING.equals(dbRef.getCollectionName());
        this.childDbRef = dbRef;
    }

    public void setChild(Child child) {
        this.child = Objects.requireNonNull(child);
        this.childDbRef = new DBRef(Child.COLLECTION_NAMING, child.id);
    }
}

public class Child {
    public static final String COLLECTION_NAMING = "Child";
    ObjectId id;
    String name;

    public Child() {
    }

    public Child(final ObjectId id, final String name) {
        this.id = Objects.requireNonNull(id);
        this.name = Objects.requireNonNull(name);
    }

    public ObjectId getId() {
        return id;
    }

    public void setId(ObjectId id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

@ApplicationScoped
public class ParentRepository {
    private final MongoDatabase mongoDatabase;

    public ParentRepository(final MongoDatabase mongoDatabase) {
        this.mongoDatabase = Objects.requireNonNull(mongoDatabase);
    }

    public Parent findByParentId(final ObjectId objectId) {
        final String loadedChild = "child_loaded";
        // Warning aggregate order is important: filter must be defined first !!
        final Bson aggregateOnParentId = match(Filters.eq("_id", objectId));
        final Bson lookupPipeline = lookup(Child.COLLECTION_NAMING, "child.$id", "_id", loadedChild);
        // I've got only on document to link with. Using the unwind avoid an array in result and create an object instead
        // You can play with this sample https://mongoplayground.net/p/bFJHVUuKrjO
        final Bson unwindPipeline = unwind("$" + loadedChild, new UnwindOptions().preserveNullAndEmptyArrays(true));
        return mongoDatabase.getCollection(Parent.COLLECTION_NAMING, Parent.class)
                .aggregate(List.of(aggregateOnParentId, lookupPipeline, unwindPipeline))
                .first();
    }
}

public class ChildCodec implements CollectibleCodec<Child> {
    private final Codec<ObjectId> objectIdCodec;

    public ChildCodec() {
        this.objectIdCodec = MongoClientSettings.getDefaultCodecRegistry().get(ObjectId.class);
    }

    @Override
    public Child generateIdIfAbsentFromDocument(final Child child) {
        if (!documentHasId(child)) {
            child.setId(new ObjectId());
        }
        return child;
    }

    @Override
    public boolean documentHasId(final Child child) {
        return child.getId() != null;
    }

    @Override
    public BsonValue getDocumentId(final Child child) {
        return new BsonObjectId(child.getId());
    }

    @Override
    public Child decode(final BsonReader reader, final DecoderContext decoderContext) {
        final Child child = new Child();
        reader.readStartDocument();
        while (reader.readBsonType() != BsonType.END_OF_DOCUMENT) {
            final String fieldName = reader.readName();
            if (fieldName.equals("_id")) {
                child.setId(objectIdCodec.decode(reader, decoderContext));
            } else if (fieldName.equals("name")) {
                child.setName(reader.readString());
            } else {
                throw new IllegalStateException("Unknown fieldName " + fieldName);
            }
        }
        reader.readEndDocument();
        return child;
    }

    @Override
    public void encode(final BsonWriter writer, final Child child, final EncoderContext encoderContext) {
        writer.writeStartDocument();
        writer.writeName("_id");
        objectIdCodec.encode(writer, child.getId(), encoderContext);
        writer.writeString("name", child.getName());
        writer.writeEndDocument();
    }

    @Override
    public Class<Child> getEncoderClass() {
        return Child.class;
    }
}

public class ParentCodec implements CollectibleCodec<Parent> {
    private final Codec<ObjectId> objectIdCodec;
    private final Codec<DBRef> dbRefCodec;
    private final Codec<Child> childCodec;

    public ParentCodec() {
        this.objectIdCodec = MongoClientSettings.getDefaultCodecRegistry().get(ObjectId.class);
        final CodecRegistry codecRegistry = CodecRegistries.fromCodecs(
                new DecodableDBRefCodec(MongoClientSettings.getDefaultCodecRegistry()),
                new ChildCodec());
        this.dbRefCodec = codecRegistry.get(DBRef.class);
        this.childCodec = codecRegistry.get(Child.class);
    }

    @Override
    public Parent generateIdIfAbsentFromDocument(final Parent parent) {
        if (!documentHasId(parent)) {
            parent.setId(new ObjectId());
        }
        return parent;
    }

    @Override
    public boolean documentHasId(final Parent parent) {
        return parent.getId() != null;
    }

    @Override
    public BsonValue getDocumentId(final Parent parent) {
        return new BsonObjectId(parent.getId());
    }

    @Override
    public Parent decode(final BsonReader reader, final DecoderContext decoderContext) {
        final Parent parent = new Parent();
        reader.readStartDocument();
        while (reader.readBsonType() != BsonType.END_OF_DOCUMENT) {
            final String fieldName = reader.readName();
            if (fieldName.equals("_id")) {
                parent.setId(objectIdCodec.decode(reader, decoderContext));
            } else if (fieldName.equals("child")) {
                if (reader.getCurrentBsonType().equals(BsonType.NULL)) {
                    reader.readNull();
                } else {
                    parent.setChild(dbRefCodec.decode(reader, decoderContext));
                }
            } else if (fieldName.equals("child_loaded")) {
                parent.setChild(childCodec.decode(reader, decoderContext));
            } else {
                throw new IllegalStateException("Unknown fieldName " + fieldName);
            }
        }
        reader.readEndDocument();
        return parent;
    }

    @Override
    public void encode(final BsonWriter writer, final Parent parent, final EncoderContext encoderContext) {
        writer.writeStartDocument();
        writer.writeName("_id");
        objectIdCodec.encode(writer, parent.getId(), encoderContext);
        writer.writeName("child");
        if (parent.getChildDbRef() != null) {
            dbRefCodec.encode(writer, parent.getChildDbRef(), encoderContext);
        } else {
            writer.writeNull();
        }
        writer.writeEndDocument();
    }

    @Override
    public Class<Parent> getEncoderClass() {
        return Parent.class;
    }
}

public class DecodableDBRefCodec extends DBRefCodec {

    public DecodableDBRefCodec(final CodecRegistry registry) {
        super(registry);
    }

    @Override
    public Class<DBRef> getEncoderClass() {
        return DBRef.class;
    }

    @Override
    public DBRef decode(final BsonReader reader, final DecoderContext decoderContext) {
        reader.readStartDocument();
        String databaseName = null;
        String ref = null;
        ObjectId id = null;
        while (reader.readBsonType() != BsonType.END_OF_DOCUMENT) {
            final String dbRefFieldName = reader.readName();
            if (dbRefFieldName.equals("$ref")) {
                ref = reader.readString();
            } else if (dbRefFieldName.equals("$id")) {
                id = reader.readObjectId();
            } else if (dbRefFieldName.equals("$db")) {
                databaseName = reader.readString();
            } else {
                throw new IllegalStateException("Unknown fieldName " + dbRefFieldName);
            }
        }
        reader.readEndDocument();
        assert ref != null;
        assert id != null;
        return new DBRef(databaseName, ref, id);
    }
}