How to create firestore transaction where target document needs to be found

Question

I am looking for a way to create a firestore transaction where i find a document from a query and then modify this document in a transaction.

Something along those lines (kotlin):

firestore.runTransaction { transaction ->

  val snapshot = transaction.get(db.collection("document")
      .whereEqualTo("someField", null)
      .orderBy("creationDate", ASCENDING)
      .limit(1L))

  val myObject = snapshot.toObject(MyObject::class.java)
  myObject.someFiled = "123"
  transaction.set(snapshot.reference, myObject)
}

The problem here is that the query returned by the .limit(1) method is not a DocumentReference, which is the only type the transaction accepts. Therefore my question is, how can such a transaction be achieved in java/kotlin?

I have seen something similar in this blog post using the admin sdk:

  return trs.get(db.collection('rooms')
    .where('full', '==', false)
    .where('size', '==', size)
    .limit(1));

Answer 1

After investigation looks like you can't do that in Kotlin/Java it is not supported. You will have to create cloud function and do something similar to:

exports.executeTransaction = functions.https.onRequest(async (req, res) => {

    const db = admin.firestore();

    const query = db
        .collection('collection')
        .where('name', '==', 'name')
        .limit(1);

    db.runTransaction(transaction => {
        return transaction
            .get(query)
            .then((querySnapshot) => {
                const gameDocSnapshot = querySnapshot.docs[0];
                const gameData = gameDocSnapshot.data();
                transaction.update(gameDocSnapshot.ref, { name: 'change' });
                return gameData;
            })
    })
        .then((gameData) => {
            res.send(gameData);
            console.log('Transaction successfully committed!', gameData);
        })
        .catch((error) => {
            res.send('Transaction failed:' + error);
            console.log('Transaction failed:', error);
        });
});