Revert "Remove data.table from class attribute"

This reverts commit 9827ff1.

r/R/metadata.R

@@ -275,11 +275,6 @@ arrow_attributes <- function(x, only_top_level = FALSE) {
     }
   }
 
-  # GH-45300: Remove data.table from class attribute
-  if (inherits(x, "data.table")) {
-    att[["class"]] <- setdiff(att[["class"]], "data.table")
-  }
-
   att <- att[setdiff(names(att), removed_attributes)]
 
   if (isTRUE(only_top_level)) {

r/tests/testthat/test-metadata.R

@@ -486,16 +486,3 @@ test_that("data.frame class attribute is not saved", {
   df_arrow <- arrow_table(df)
   expect_identical(df_arrow$r_metadata, list(attributes = list(foo = "bar"), columns = list(x = NULL)))
 })
-
-test_that("we remove the data.table class from metadata", {
-  skip_if_not_installed("data.table")
-
-  tf <- tempfile()
-  on.exit(unlink(tf))
-
-  dt <- data.table::data.table(x = 1:3)
-  write_parquet(dt, tf)
-  dt_after <- read_parquet(tf)
-
-  expect_false("data.table" %in% attributes(dt_after)$class)
-})