FR: allow DocumentReference.select similar to Query.select

[goldensunliu]

Hey guys,
I have been snooping around the source code and documentation. It seems like the only way to do apply a field mask to the result and returns only the specified subset of fields is through the Query.select method.
https://cloud.google.com/nodejs/docs/reference/firestore/0.8.x/Query#select
https://github.com/googleapis/nodejs-firestore/blob/master/src/reference.js#L1080

Is there any plan or ways to enable the same feature when getting a single document via DocumentReference? The code seems plenty of references to https://cloud.google.com/firestore/docs/reference/rest/v1beta1/DocumentMask which is used to restrict a get or update operation on a document to a subset of its fields, but I didn't have any luck tracking down usage of this in DocumentReference.get

Relevant Code:

// based on the select example at https://cloud.google.com/nodejs/docs/reference/firestore/0.8.x/Query#select
let collectionRef = firestore.collection('col');
let documentRef = collectionRef.doc('doc');

documentRef.set({x:10, y:5}).then(() => {
  return collectionRef.where('x', '>', 5).select('y').get();
}).then((res) => {
  console.log(`y is ${res.docs[0].get('y')}.`);
});
// would like to be able to also do
documentRef.select('y').get().then((snapShot) => {
  console.log(`snapShot only has one field ${snapShot}`);
  // Expected output: {y: 5}
});

[goldensunliu]

I have also filed this against firebase/firebase-js-sdk#272 since I am not sure where the source of development on the firestore part is housed.

[goldensunliu]

firebase/firebase-js-sdk#212 answers this

[IchordeDionysos]

@goldensunliu Are you sure that that the issue in the Firebase JS SDK really answers this?
As I understood the answer from @wilhuff, is that this won't appear in the JS CLIENT SDK as you are having offline capabilities there and so on.

I don't see a problem in the ADMIN SDK to add such a method to the DocumentReference. As far as I know, there is no caching in the ADMIN SDK, so it should be no problem to allow a snippet/method like this:

admin.firestore().document("path/to/a/document").select("y").get().then(snap => {
    console.log(`snapShot only has one field ${snap.data()}`);
    // Expected output: {y: 5}
});

Or am I wrong?

[wilhuff]

@IchordeDionysos you're right: the comments in firebase/firebase-js-sdk#212 specifically relate to the mobile clients and the complications of interacting with the cache while offline. These concerns do not apply to the nodejs or admin SDKs.

document(..).get() translates into a BatchGetDocumentsRequest RPC, which includes a document mask, so this should be possible to implement.

In the meantime you can work around this by writing the following instead:

const projectId = // ... your project id 
const path = 'path/to/a/document';
db.document(path)
    .parent  // i.e. the collection containing the document
    .where('__name__', '==', `projects/${projectId}/databases/(default)/documents/${path}`)
    .select(...)

Note that the result will be a query snapshot rather than a document snapshot, but it should get you what you want in the short term.

@schmidt-sebastian holler if I got this wrong.

[schmidt-sebastian]

A more concise version would be as follows:

const docRef = db.doc('path/to/a/document');
docRef.parent.where(Firestore.FieldPath.documentId(), '==', docRef).select(...)

[triplef]

It would be great to have this issue re-opened for consideration.

As mentioned above, the comments in firebase/firebase-js-sdk#212 don’t apply to the Admin SDK. It would be great to have a DocumentReference.select() function implemented there.

[schmidt-sebastian]

We are currently debating on how to best implement this. Please stay tuned.