collection method
Gets a CollectionReference for the specified Firestore path.
Implementation
CollectionReference<Map<String, dynamic>> collection(String collectionPath) {
if (collectionPath.isEmpty) {
throw ArgumentError('A collection path must be a non-empty string.');
} else if (collectionPath.contains('//')) {
throw ArgumentError('A collection path must not contain "//".');
} else if (!isValidCollectionPath(collectionPath)) {
throw ArgumentError(
'A collection path must point to a valid collection.',
);
}
return _JsonCollectionReference(this, _delegate.collection(collectionPath));
}