Getting second last record from firestore DB, javascript

Question

As firestore is not that much old yet, there is very small help out there in some scenarios.

I am trying to get 2nd last record from Firestore but it's getting very first one. i know it's wrong query:

db.collection("batches")
  .where('added_at', "<", paymentData.added_at) //Here paymentData is last record
  .limit(1).get().then(function(prevSnapshot){
})

For example i have 3 records

A. added_at: 1 Jan, 2017
B. added_at: 2 Jan, 2017
C. added_at: 3 Jan, 2017

And i have C in paymentData right now and i want record B. But it's getting record A.

How do i get Second Last Record ?

EDIT:

Each record will always have newer timestamp than previous one (even it can be 1 Minute. e.g previous 27 Jan, 2017 5:20 PM and new 27 Jan, 2017 5:21 PM)

And paymentData will always have latest record.

Basically i want to compare two values of current payment and previous payment and display it's difference to user.

Answer 1

The order of Firestore documents is not guaranteed, unless you specifically order the documents in your query. To do this, you'll need to use the orderBy() method.

For example, you can specify that the database sorts your documents by the added_at field in descending order, which should return your desired result, B:

db.collection("batches")
  .orderBy('added_at', 'desc') // Order documents by added_at field in descending order
  .where('added_at', "<", paymentData.added_at)
  .limit(1).get().then(function(prevSnapshot){
          // ...
  })

Be careful though: multiple documents with the same timestamp, or timestamps in between the one you're expecting, may mean that you'll receive a different document than expected, so I wouldn't rely on this to pick specific documents.