Transform a Mongoose Document object's ObjectId fields to strings. This transformation is necessary, in order to use class-transformer's plainToClass function on documents with ObjectId fields. class-transformer fails and throws an error when converting the BSON ObjectId type to a Buffer.

extract-mongo-id.ts

function extractMongoId(document) {
  const doc = document.toObject ? document.toObject() : document;
  for (const [key, value] of Object.entries(doc)) {
    if (value instanceof ObjectID) {
      delete doc[key];
      doc[key] = value.toHexString();
    } else if (value instanceof Object) {
      doc[key] = extractMongoId(value);
    }
    if (Array.isArray(value)) {
      doc[key] = value.map(extractMongoId);
    }
  }
  return doc;
}

// Usage
plainToClass(targetType, Array.isArray(document) ? document.map(extractMongoId) : extractMongoId(document), transformOptions);