Firestore Transaction get() returning QuerySnapshot but it should be DocumentSnapshot

[Rubenazo]

Environment

• Operating System version: Windows 7
• Firebase SDK version: 4.2.0
• Library version: 6.0.0
• Firebase Product: firestore

The problem

I'm trying to reproduce a firestore transaction with this sample code from the firebase documentation https://firebase.google.com/docs/firestore/manage-data/transactions?hl=es-419

// Initialize document
var cityRef = db.collection('cities').doc('SF');
var setCity = cityRef.set({
  name: 'San Francisco',
  state: 'CA',
  country: 'USA',
  capital: false,
  population: 860000
});

var transaction = db.runTransaction(t => {
  return t.get(cityRef)
      .then(doc => {
        // Add one person to the city population
        var newPopulation = doc.data().population + 1;
        t.update(cityRef, { population: newPopulation });
      });
}).then(result => {
  console.log('Transaction success!');
}).catch(err => {
  console.log('Transaction failure:', err);
});

However in db.runTransaction(t => { return t.get(cityRef).then(doc => { ... } ... it is say that doc must return a DocumentSnapshot, but it returns a QuerySnapshot. Even that way i can't obtain the DocumentSnapshot from doc, so there is a problem with the documentation and i'm not sure if there is also a bug with the return value.

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[hiranya911]

Your code compiles and works just fine for me. Here it is with some TypeScript annotations added for clarity (this compiles successfully):

    const db = admin.firestore();
    const cityRef: admin.firestore.DocumentReference = db.collection('foo').doc('bar');
    var transaction = db.runTransaction(t => {
      return t.get(cityRef)
          .then((doc: admin.firestore.DocumentSnapshot) => {
            // Add one person to the city population
            var newPopulation = doc.data().population + 1;
            t.update(cityRef, { population: newPopulation });
          });
    }).then(result => {
      console.log('Transaction success!');
    }).catch(err => {
      console.log('Transaction failure:', err);
    });

t.get() also has an overload that returns a Promise<QuerySnapshot>, but that only gets invoked if get() was called with a Query.

[hiranya911]

One possibility is you're running the transaction on a collection reference, which extends Query.

[Rubenazo]

@hiranya911 Thanks, i first made sure i was running the transaction in a document reference and it was indeed a document reference, so i finally made it work by explicitly declaring the types for DocumentReference and DocumentSnapshot as you did. I had my code inside a function so i did it like this:

async function setCounts(storeRef: admin.firestore.DocumentReference) { // Type here

  var transaction = admin.firestore().runTransaction(t => {
    return t.get(storeRef)
    .then((doc: admin.firestore.DocumentSnapshot) => { // Type here
      x = doc.data(); // Now i can get data() from the doc
    });
  }).then(result => {
    console.log('Transaction success!');
  }).catch(error => {
    console.log('Transaction failure:', error);
  });

}

I don't know why, but while deploying the linter inferred doc as QuerySnapshot even though it was not. There may be something to consider. Thanks again.

[schmidt-sebastian]

Transaction.get() returns a DocumentSnapshot if you pass a DocumentReference (https://github.com/googleapis/nodejs-firestore/blob/master/dev/src/transaction.js#L89) and a QuerySnapshot if you pass a Query or a CollectionReference (https://github.com/googleapis/nodejs-firestore/blob/master/dev/src/transaction.js#L97). Everything else should get rejected.

Please let us know if you still encounter this issue. I am closing this as it looks like it is working for you now.