How can I insert a vertex only if it doesn't yet exist?

[gfaraj]

I'm doing this which I thought would do it, but I'm still getting an error:

await _source.V(id)
    .Fold()
    .Coalesce(__ => __.Unfold().AddV(new User { Id = id }))
    .FirstOrDefaultAsync();

[2022-10-19T15:12:29.873Z] Errors : [
[2022-10-19T15:12:29.873Z] "Resource with specified id or name already exists."
[2022-10-19T15:12:29.878Z] ]
[2022-10-19T15:12:29.878Z] );
[2022-10-19T15:12:29.878Z] StatusCode = Conflict;
[2022-10-19T15:12:29.878Z] SubStatusCode = 0;
[2022-10-19T15:12:29.878Z] ActivityId = 4ffd1e94-732f-4645-8cc4-e02b3764b634;

Any ideas what I'm doing wrong?

[gfaraj]

I was misusing Coalesce. This worked:

await _source.V(id)
    .Fold()
    .Coalesce(__ => __.Unfold(), __ => __.AddV(new User { Id = id }))
    .FirstOrDefaultAsync();