Google-cloud-go: firestore: document how to tell "not exists" error from other errors in DocumenRef.Get()

[kjk]

From https://godoc.org/cloud.google.com/go/firestore#example-DocumentRef-Get, example for Get:

ctx := context.Background()
client, err := firestore.NewClient(ctx, "project-id")
if err != nil {
    // TODO: Handle error.
}
defer client.Close()

docsnap, err := client.Doc("States/Ohio").Get(ctx)
if err != nil {
    // TODO: Handle error.
}

In JavaScript SDK, result of Get() has exists field.

As far as I can tell in Go SDK I can't distinguish "document doesn't exist" error from e.g. network connection error.

I hacked a heuristic function:

func dbIsNotFoundError(err error) bool {
    s := err.Error()
    return strings.Contains(s, "code = NotFound")
}

but I would prefer a supported and documented way of doing that.

[jba]

Try

import (
    "google.golang.org/grpc"
    "google.golang.org/grpc/codes"
)

if grpc.Code(err) == codes.NotFound

[kjk]

Thanks, I've tested it and it worked.

That being said, I imagine it's a common thing to do and impossible to figure out hence 2 suggestions:

• expand https://godoc.org/cloud.google.com/go/firestore#example-DocumentRef-Get doc to show this pattern (as well https://firebase.google.com/docs/firestore/query-data/get-data which was my primary documentation when learning the SDK)
• add a helper function so that I can call e.g. firestore.IsNotFoundError(err). It's not to save few import lines but to improve discoverability of this functionality

[makuc]

However, if we determene existence of a document based on an error, why do we even have a method Exists() on type DocumentSnapshot?

[Setheck]

seems kind of redundant to use the Exists() method.
Since you have to call Get() handle a possible error, then call the Exists() method. If the error from the Get() is a not found you would need to explicitly ignore it then call Exists()
am i missing something?

[tbpg]

I can imagine situations where it's more convenient to call an Exists() method. But, that's a good question. :)

If Get() returns an error with grpc.Code(err) == codes.NotFound, you don't have to call Exists(). At that point, you already know the document you were trying to get does not exist.

[hoshitocat]

grpc.Code(err) is deprecated.
but, we can use status.Code(err).
https://pkg.go.dev/google.golang.org/grpc/status#Code

import (
    "google.golang.org/grpc/codes"
    "google.golang.org/grpc/status"
)

// deprecated
// if grpc.Code(err) == codes.NotFound
if status.Code(err) == codes.NotFound