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