[Flutter] Flutter - How to add a field to all documents in firebase collection

I am using flutter firebase. And I want a query to add a key and value to all documents in firebase collection.

I try to use batch write but it add a new documents having field. But not merge to existing documents.

var db= Firestore.instance();
var batch = db.batch();
batch.setData(
db.collection("users").document(),
{"status": "Approved"}
);


When I try to give document Id like document('id') it add only to that document.

I try many and watches YouTube videos but not able find still now. Please help me !

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