Not sure why we need to do that much. Tests seem to pass if we only remove the skip() call?

To run an individual test file, use:

testthat::test_local(filter = "^relational$")

I was mistaken: we can remove the skip() call on the latest version of your branches combined. But we could do that separately, I still don't think changing the tests should be necessary at all.

@krlmlr I don't want to change the tests, but without the change in this PR (i.e as.data.frame.duckdb_relation) we continue to get the following error during CI. Do you have an idea as to why this is?

I've seen this too in #6048, but not when applying to your latest WIP PRs. Maybe it's worth enabling the tests in one of those?

The error happens in CI or when you run the tests on an ubuntu image. I can't recreate the error locally either. It makes me think that there has been some update somewhere in the dependencies where we now have to specify the extra .duckdb_relation