Firestore transaction update multiple collections in a single transaction

Question

How can i update multiple collections in firestore by using a single transaction i searched but i didn't get any answers. Is it possible to update multiple collections in a single transaction?

I want to update branch.name at-once in classroom & students collection

Answer 1

As explained in the documentation for the Node.js Client Library update() method, it returns a Transaction which is " used for chaining method calls ." (Note that update() method of the Admin SDK behaves exactly the same way).

So, for example, if within the transaction you want to get a value from a classroom doc, increase it and use it to update two documents from two different collections ( classrooms and students ), you would do as follows:

const db = firebase.firestore();  //or admin.firestore() in a Cloud Function
const docRef1 = db.collection('classrooms').doc('classroomDocId');
const docRef2 = db.collection('students').doc('studentDocId');


let transaction = db.runTransaction(t => {
  let newNumericValue;
  return t.get(docRef1 )
    .then(doc => {
      newNumericValue = doc.data().numericValue + 1;  //You calculate the new value
      return t.update(docRef1 , {numericValue : newNumericValue});
    }).then(t => {
      return t.update(docRef2 , {numericValue : newNumericValue});
    });
}).then(result => {
  console.log('Transaction success!' + result);
}).catch(err => {
  console.log('Transaction failure:', err);
});

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Note that if you need to do several reads before your multiple updates, "when using transactions, read operations must come before write operations ."

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On the other hand, IF YOU JUST WANT TO UPDATE multiple documents WITHOUT READING one or more values (you say in your question that you "want to update branch.name at-once in classroom & students collection "), you don't need to use a transaction. Just use a batched write , as follows:

let batch = db.batch();

let cRef = db.collection('classrooms').doc('classroomDocId');
batch.set(cRef, {branch.name: 'newName'});

let sRef = db.collection('students').doc('studentDocId');
batch.update(sRef, {branch.name: 'newName'});

return batch.commit().then(function () {
  // ...
});

---

UPDATE Following your comments

In your Cloud Function, you can very well chain different Firestore queries (with where() ) and in each then() populate the batch and then in the last then() commit the batch. See below for an example (just adapt with the correct queries):

 let batch = admin.firestore().batch();

 return admin.firestore().collection('students').where('branch.id', '==', documentId).get()
 .then((querySnapshot) => {
    querySnapshot.forEach((doc) => { 
        batch.update(doc.ref, {branch: {id: documentId, name: after.name}}); 
    });
    return admin.firestore().collection('student_public').where('branch.id', '==', documentId).get();
 })
 .then((querySnapshot) => {
    querySnapshot.forEach((doc) => { 
        batch.update(doc.ref, {branch: {id: documentId, name: after.name}}); 
    });
    return batch.commit() 
 })
 .catch(err => { console.log('error===>', err); });