indexing doesn't work with object fields

[xh-lin]

After indexing a field that's an object type, the value will become empty after inserting.
I cannot use objects for the _id field because of the same issue
I believe the issue is maybe in this function

react-native-local-mongodb/lib/indexes.js

Lines 13 to 34 in b5955be

	/**
	* Type-aware projection
	*/
	function projectForUnique(elt) {
	if (elt === null || Array.isArray(elt)) {
	return '$null';
	}
	if (typeof elt === 'string') {
	return '$string' + elt;
	}
	if (typeof elt === 'boolean') {
	return '$boolean' + elt;
	}
	if (typeof elt === 'number') {
	return '$number' + elt;
	}
	if (util.isArray(elt)) {
	return '$date' + elt.getTime();
	}
	return elt; // Arrays and objects, will check for pointer equality
	}

export function testIndex(): void {
  db.loadDatabase();
  db.remove({}, { multi: true });

  db.ensureIndex({ fieldName: 'key' }, (err) => {
    if (err != null) LOG.error('ensureIndex err:', err);
  });

  db.insert({ key: { a: 1 } }, (err, doc) => {
    if (err != null) LOG.error('a1:', err);
    else LOG.info('a1 doc added:', doc);
  });
  db.insert({ key: { a: 2 } }, (err, doc) => {
    if (err != null) LOG.error('a2:', err);
    else LOG.info('a2 doc added:', doc);
  });
  db.insert({ key: { b: 1 } }, (err, doc) => {
    if (err != null) LOG.error('b1:', err);
    else LOG.info('b1 doc added:', doc);
  });
  db.insert({ key: { b: 2 } }, (err, doc) => {
    if (err != null) LOG.error('b2:', err);
    else LOG.info('b2 doc added:', doc);
  });

  db.count({}, (_, count) => {
    LOG.info(`count: ${count}`);
  });
}