How to avoid double Cloud Firestore write when uploading Storage file using the document ID and passing the DownloadURL

Question

I am working on a Flutter app connected to Firebase Cloud Firestore and Storage, and I want to create documents in Cloud Firestore that contain the downloadURL of files in Storage. However, I also need the filename to be the unique ID of the same Firestore document. Right now I upload the file after I create the document, but it would cost me an extra write to update the Firestore document afterwards.

I want to prevent this. It might work if I could access the id of the newly created document within the add() function, so that I can pass it to the storageTask, which would return the downloadURL directly inside the add() function. Another option would be if I could create a document, get its ID, then do the upload task, and write all the data afterwards, but I''m not sure if this would count as one write or two, nor do I know how to create a document without setting any data.

What I have now is roughly like this:

CollectionReference activities = FirebaseFirestore.instance.collection('activities');
activities.add({
    'postDate': DateTime.now(),
}).then((value) => _startUpload(id: value.id));

Where _startUpload is a function that uploads a file to Storage, and could potentially return a downloadURL. I want to be able to access that URL in the add function like this:

CollectionReference activities = FirebaseFirestore.instance.collection('activities');
activities.add({
    'postDate': DateTime.now(),
    'downloadURL': _startUpload(id: SomehowGetThisDocuments.id)
});

marvinsxtr marvinsxtr · Accepted Answer · 2020-12-11T21:37:40

You can do something like described in the last paragraph of this section in the documentation:

newActivityRef = activities.document();
newActivityRef.set({
    'postDate': DateTime.now(),
    'downloadURL': _startUpload(id: newActivityRef.documentID)
});

This should count as one write as .document().set() is equivalent to .add() according to the documentation.

How to avoid double Cloud Firestore write when uploading Storage file using the document ID and passing the DownloadURL

1 Answers