Firebase Transaction MutableData showing Nil

Question

I have a path that I run a Transaction on. I check to see if the value is nil before proceeding. If it is nil I abort and if not I continue. The issue is the value isn't nil but the mutable data is saying it is.

db:

@posts
   @postId_abc
      -date: ....
      -views: 0

Transaction:

let viewsPath = Database.database().reference()
                   .child("posts")
                   .child("postId_abc")
                   .child("views")

viewsPath.runTransactionBlock({ (mutableData: MutableData) -> TransactionResult in

    let doesViewsExist = mutableData.value as? Int // I also tried Double
    if doesViewsExist == nil {
                        
        return TransactionResult.abort() // "views" value is 0
    }

    // ...
}

score 0 · Accepted Answer · answered Mar 24 '21 at 22:26

This is the expect behavior. When you start a transaction the SDK immediately calls your transaction handler with its best guess of the current value of the node. Most often that guess will be nil, as the client has no better knowledge yet.

So you will have to not abort the transaction, but instead set another value on the mutable data, for example 0. It doesn't really matter much what that value is, as the transaction will be retried anyway.

For more background on this, see:

Firebase realtime database transaction handler gets called twice most of the time, with an ASCII chart of the flow
Firebase runTransaction not working - MutableData is null
how to handle null values on firebase transaction api

Firebase Transaction MutableData showing Nil

1 Answers1