I added the Autocomplete via a PR, without it there is no way to attach existing records

also strange that it's adding an Autocomplete at all, since the field is a has_one 🙂

I've always understood the HasOneField to be a GridField with 0 or 1 rows

might be, as I don’t see that being backed up by actual data model, which is why it can’t be a datalist (wild guess) (edited)

Seems HasOneButtonRelationList is an ArrayList rather than a DataList?

has anyone come across this using the HasOneField? ERROR [Emergency]: Uncaught TypeError: Argument 1 passed to SilverStripe\ORM\DataList::subtract() must be an instance of SilverStripe\ORM\DataList, instance of SilverShop\HasOneField\HasOneButtonRelationList given, called in /home/nope/nope/nope/vendor/silverstripe/framework/src/Forms/GridField/GridFieldAddExistingAutocompleter.php