in my android application , i need to delete to all documents inside "cart" collection. but my code is not working.
mStore.collection("Users").document(fAuth.getCurrentUser().getUid())
.collection("Cart").document().delete();
Please note that your question has been answered before.
You can't easily delete all documents of a collection in Firestore, see docs. You have to get all documents of the collection and delete them individually.
To delete an entire collection or subcollection in Cloud Firestore, retrieve all the documents within the collection or subcollection and delete them. If you have larger collections, you may want to delete the documents in smaller batches to avoid out-of-memory errors. Repeat the process until you've deleted the entire collection or subcollection.
You should do this server-side, for example by invoking a Cloud Function that then deletes all documents of a collection. The biggest problem doing this on the client is dealing with large collections.
Deleting a collection requires coordinating an unbounded number of individual delete requests. If you need to delete entire collections, do so only from a trusted server environment. While it is possible to delete a collection from a mobile/web client, doing so has negative security and performance implications.
TL&DR: Get all documents of a collection, delete them individually. Preferrably on a server.
