i want to use mongo collection insertMany() to add all tags data at once to avoid multiple network calls

Question

@Override
public Question createQuestion(Question question) {
    var existingQuestion = questionRepository.findByQuestionAndCourseAndLevelAndOptions(question.getQuestion(), question.getCourse(), question.getLevel().toString(), question.getOptions());
    if (existingQuestion == null || !existingQuestion.equals(question)) {
        var savedQuestion = questionRepository.save(question);
        if (savedQuestion != null) {
            var tags = savedQuestion.getTags();
            List<Tag> newTags = new ArrayList<>();
            List<UpdateOneModel<Tag>> updateModels = new ArrayList<>();

            MongoCollection tagCollection = ((MongoCollection) tagRepository);
            tags.forEach(tagName -> {
                Document byTagName = (Document) tagCollection.find(eq("tagName", tagName)).first();
                if (byTagName != null) {
                    updateModels.add(new UpdateOneModel<>(
                            eq("_id", byTagName.getClass()),
                            addToSet("questions", savedQuestion.getId())
                    ));
                } else {
                    newTags.add(new Tag(tagName, Collections.singleton(savedQuestion.getId())));
                }
            });

            if (!newTags.isEmpty()) {
                tagCollection.insertMany(newTags);
            }

            if (!updateModels.isEmpty()) {
                tagCollection.bulkWrite(updateModels);
            }

                 if (!newTags.isEmpty()) {
                    tagRepository.saveAll(newTags);
                }
        }
        return question;
    }
    return null;
}

i am trying to add all tags data at once in mongo db using insertMany() but getting classCastException