Fix ref_grid with missing levels

[banfai]

Closes #508

Solution from https://stackoverflow.com/a/78719672

[rvlenth]

OK, I have look at this as well as the tests in #500, and think this is the right fix for the problem.

[banfai]

thanks a lot for the quick review

[rvlenth]

PS -- I added tests for the case where we allow factors to have NA levels:

# Missing and NA levels
miss.df = data.frame(x = factor(c("a", "a", "b", NA), levels = c("a", "b", "c", NA)), y = 1:4)
miss.lm = lm(y ~ x, data = miss.df)
miss.rg1 = ref_grid(miss.lm)
miss.rg2 = ref_grid(miss.lm, data = miss.df)
# Now try allowing NA levels
miss.dfa = transform(miss.df, x = factor(x, exclude = NULL))
miss.lma = lm(y ~ x, data = miss.dfa)
miss.rg1a = ref_grid(miss.lma)
miss.rg2a = ref_grid(miss.lma, data = miss.dfa)
test_that("Reference grid handles missing values", {
    expect_equal(length(miss.rg1@levels$x), 2)
    expect_equal(length(miss.rg2@levels$x), 2)
    expect_equal(length(miss.rg1a@levels$x), 3)
    expect_equal(length(miss.rg2a@levels$x), 3)
    emm_options(allow.na.levs = FALSE)
    expect_error(ref_grid(miss.lma))
    emm_options(allow.na.levs = NULL) # revert to default
})

Happy to say they all pass!

Thanks for the contribution!

[rvlenth]

Please confirm that this is correct for you in the DESCRIPTION file:

    person("Balasz", "Banfai", role = "ctb"),

[banfai]

Please confirm that this is correct for you in the DESCRIPTION file:

z and s are the other way around:

    person("Balazs", "Banfai", role = "ctb"),

PS -- I added tests for the case where we allow factors to have NA levels:
Happy to say they all pass!
Thanks for the contribution!

That's great, thanks for expanding the tests. Happy to contribute to this amazing package!