Firebase: Transaction read and update multiple documents

Question

With this code I can read and update single document in the transaction.

// Update likes in post
var docRef = admin
  .firestore()
  .collection(\"posts\")
  .do         


        

Answer 1

You can use batch to accomplish what you want.

var batch = db.batch();

var docRef = admin
  .firestore()
  .collection("posts")
  .doc(doc_id);

var likes = ... // get the amount of likes

batch.update(docRef, { likes })

var profileRef = admin
  .firestore()
  .collection('profile')
  .doc(profId)
batch.update(profileRef, { likes })

batch.commit() // at this point everything works or not
  .then(() => {
    console.log('success!')
  })
  .catch(err => {
    console.log('something went wrong')
  })

Answer 2

To update multiple documents in a transaction, you call t.update() multiple times.

let promise = await admin.firestore().runTransaction(transaction => {
  var post = transaction.get(docRef);
  var anotherPost = transaction.get(anotherDocRef);

  if (post.exists && anotherPost.exists) {
    var newLikes = (post.data().likes || 0) + 1;
    await transaction.update(docRef, { likes: newLikes });
    newLikes = (anotherPost.data().likes || 0) + 1;
    await transaction.update(anotherdocRef, { likes: newLikes });
  }
})

See https://firebase.google.com/docs/firestore/manage-data/transactions#transactions