mongimport upsert creates new documents - mongodb

Question

When I try to execute a mongoimport with upsertFields like so:
> mongoimport --db upsert-test --collection data --type tsv --headerline --file upsert-data.tsv --upsertFields MyCustomUpsertField -vvv
2018-10-10T15:08:39.358+0200 using upsert fields: [MyCustomUpsertField]
2018-10-10T15:08:39.424+0200 using 8 decoding workers
2018-10-10T15:08:39.424+0200 using 1 insert workers
2018-10-10T15:08:39.425+0200 will listen for SIGTERM, SIGINT, and SIGKILL
2018-10-10T15:08:39.425+0200 filesize: 61 bytes
2018-10-10T15:08:39.426+0200 using fields: "MyCustomUpsertField","SomeData"
2018-10-10T15:08:39.431+0200 connected to: localhost
2018-10-10T15:08:39.431+0200 ns: upsert-test.data
2018-10-10T15:08:39.431+0200 connected to node type: standalone
2018-10-10T15:08:39.432+0200 standalone server: setting write concern w to 1
2018-10-10T15:08:39.432+0200 using write concern: w='1', j=false, fsync=false, wtimeout=0
2018-10-10T15:08:39.432+0200 standalone server: setting write concern w to 1
2018-10-10T15:08:39.432+0200 using write concern: w='1', j=false, fsync=false, wtimeout=0
2018-10-10T15:08:39.433+0200 got line: ["Upsert-ID-1" "SomeData1"]
2018-10-10T15:08:39.433+0200 imported 1 document
And then execute the same command again, The result is two documents with exactly the same data.
Adding the (supposedly obsolete) --mode upsert flag changes nothing. New documents are always created.
I was under the impression, that upserFields would search for already existing documents with MyCustomUpsertField == "Upsert-ID-1" and update those documents instead of creating new ones?
Env Info
> mongo --version
MongoDB shell version v4.0.0
git version: 3b07af3d4f471ae89e8186d33bbb1d5259597d51
allocator: tcmalloc
modules: none
build environment:
distmod: 2008plus-ssl
distarch: x86_64
target_arch: x86_64
> mongoimport --version
mongoimport version: r4.0.0
git version: 3b07af3d4f471ae89e8186d33bbb1d5259597d51
Go version: go1.8.5
os: windows
arch: amd64
compiler: gc
OpenSSL version: OpenSSL 1.0.2o-fips 27 Mar 2018
What am I doing wrong?

Answer 1

You have an issue related to quotes in your TSV header which is similar to this one: https://jira.mongodb.org/browse/TOOLS-61
When you look at your screenshot above you'll notice that your field names are not MyCustomUpsertField but "MyCustomUpsertField" - with the quotes included.
So what you want to do is either remove the quotes from your file (I would strongly suggest that because it looks funky on a JSON level and I feel this is going to cause issues somewhere) or find a way to use quotes in the command line, kind of like this:
mongoimport --db upsert-test --collection data --type tsv --headerline --file upsert-data.tsv --upsertFields "MyCustomUpsertField" -vvv
Mind you, I haven't tried the above and would guess it won't behave as expected.