Firestore: Reading multiple documents in transaction - swift

Question

When I run the Firestore transaction it gives me the error :
"Every document read in a transaction must also be written in that transaction."
But all the documents that I am trying to read in the transaction are also being written to in the code below ?
db.runTransaction({ (transaction, errorPointer) -> Any? in
let doc1: DocumentSnapshot
let doc2: DocumentSnapshot
let doc3: DocumentSnapshot
do {
try doc1 = transaction.getDocument(self.doc1Ref)
try doc2 = transaction.getDocument(self.doc2Ref)
try doc3 = transaction.getDocument(self.doc3Ref)
} catch let fetchError as NSError {
errorPointer?.pointee = fetchError
return nil
}
// WRITES TO DOC1, DOC2, DOC3
guard let userPostCount = doc1.data()?["postCount"] as? Int,
let locationPostCount = doc2.data()?["postCount"] as? Int,
let itemPostCount = doc3.data()?["postCount"] as? Int,
let locationAvgRating = doc2.data()?["averageRating"] as? Float,
let itemAvgRating = doc3.data()?["averageRating"] as? Float else { return nil }
// Create post on another document not in transaction
transaction.setData(post.dictionary, forDocument: self.doc4Ref)
// Update counts for #userPosts, #locationPosts, #itemPosts, avgLocationRating, avgItemRating
transaction.updateData(["postCount": userPostCount + 1], forDocument: self.doc1Ref)
let newAvgLocationRating = ((avgLocationRating * Float(locationPostCount)) + Float(post.rating)) / (Float(locationPostCount) + 1.0)
transaction.updateData(["postCount": locationPostCount + 1, "averageRating": newAvgLocationRating], forDocument: self.doc2Ref)
let newAvgItemRating = ((avgItemRating * Float(itemPostCount)) + Float(post.rating)) / (Float(itemPostCount) + 1.0)
transaction.updateData(["postCount": locationPostCount + 1, "averageRating": newAvgItemRating], forDocument: self.doc3Ref)
// Add postID to user and location
transaction.setData(["postID": self.postRef.documentID, "locationID": post.locationID, "itemID": post.itemID, "rating": post.rating, "timestamp": post.timestamp], forDocument: self.doc1Ref.collection("posts").document(self.postRef.documentID))
transaction.setData(["postID": self.postRef.documentID, "rating": post.rating, "timestamp": post.timestamp], forDocument: self.doc3Ref.collection("posts").document(self.postRef.documentID))
return nil
}) { (object, error) in
if let error = error {
print(error)
} else {
print("done")
}
}
Is it just not possible to do multiple .getDocument(documentReference) in a transaction?
Is there another way to accomplish this issue?

Answer 1

you should put all your if operations in do and else operations in catch
don't use them outside the block.
thats what the error is saying "Every document read in a transaction must also be written in that transaction.".
for ex. your code
// WRITES TO DOC1, DOC2, DOC3
guard let userPostCount = doc1.data()?["postCount"] as? Int,
let locationPostCount = doc2.data()?["postCount"] as? Int,
let itemPostCount = doc3.data()?["postCount"] as? Int,
let locationAvgRating = doc2.data()?["averageRating"] as? Float,
let itemAvgRating = doc3.data()?["averageRating"] as? Float else { return nil }
is doing operations with doc1,doc2 outside the scope of do catch block

Answer 2

I just noticed you are using Apple's swift scripting. My answer is standards based JavaScript but I hope it helps you. I don't use or know anything about Swift...
As far as reading data from 3 documents simultaneously, the way to do this is using javascript's .promise.all. While the below code is not precisely your solution, you can get the just of how to do it.
You basically push each .get into an array then you .promise.all that array .then you iterate over the returned data.
try {
targetRef.get().then(function(snap) {
if (snap.exists) {
for (var key in snap.data()) {
if (snap.data().hasOwnProperty(key)) {
fetchPromise = itemRef.doc(key).get();
fetchArray.push(fetchPromise);
}
}
Promise.all(fetchArray).then(function(values) {
populateSelectFromQueryValues(values,"order-panel-one-item-select");
if(!selectIsEmpty("order-panel-one-item-select")){
enableSelect("order-panel-one-item-select");
}
});
}
}).catch(function(error) {
toast("error check console");
console.log("Error getting document:", error);
});
}