From 4c413c39dad6fe703bbc519f6332e3496dff091c Mon Sep 17 00:00:00 2001
From: Romain Francois <romain@rstudio.com>
Date: Thu, 29 Oct 2020 12:17:28 +0100
Subject: [PATCH 1/2] store metadata for each element of a list column too, not
 just the list itself. ARROW-10386.

---
 r/R/record-batch.R               | 20 ++++++++++++++------
 r/R/table.R                      | 19 ++++++++++++++++---
 r/tests/testthat/test-metadata.R |  7 +++++++
 3 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/r/R/record-batch.R b/r/R/record-batch.R
index 331a7a77253..2d20a93e85f 100644
--- a/r/R/record-batch.R
+++ b/r/R/record-batch.R
@@ -286,6 +286,20 @@ as.data.frame.RecordBatch <- function(x, row.names = NULL, optional = FALSE, ...
 
 apply_arrow_r_metadata <- function(x, r_metadata) {
   tryCatch({
+    columns_metadata <- r_metadata$columns
+    if (is.data.frame(x)) {
+      if (length(names(x)) && !is.null(columns_metadata)) {
+        for (name in intersect(names(columns_metadata), names(x))) {
+          x[[name]] <- apply_arrow_r_metadata(x[[name]], columns_metadata[[name]])
+        }
+      }
+    } else if(is.list(x) && !inherits(x, "POSIXlt") && !is.null(columns_metadata)) {
+      x <- map2(x, columns_metadata, function(.x, .y) {
+        apply_arrow_r_metadata(.x, .y)
+      })
+      x
+    }
+
     if (!is.null(r_metadata$attributes)) {
       attributes(x)[names(r_metadata$attributes)] <- r_metadata$attributes
       if (inherits(x, "POSIXlt")) {
@@ -297,12 +311,6 @@ apply_arrow_r_metadata <- function(x, r_metadata) {
       }
     }
 
-    columns_metadata <- r_metadata$columns
-    if (length(names(x)) && !is.null(columns_metadata)) {
-      for (name in intersect(names(columns_metadata), names(x))) {
-        x[[name]] <- apply_arrow_r_metadata(x[[name]], columns_metadata[[name]])
-      }
-    }
   }, error = function(e) {
     warning("Invalid metadata$r", call. = FALSE)
   })
diff --git a/r/R/table.R b/r/R/table.R
index 1d2190589f7..172e7bceab7 100644
--- a/r/R/table.R
+++ b/r/R/table.R
@@ -210,11 +210,24 @@ arrow_attributes <- function(x, only_top_level = FALSE) {
 
   if (is.data.frame(x)) {
     columns <- map(x, arrow_attributes)
-    if (length(att) || !all(map_lgl(columns, is.null))) {
+    out <- if (length(att) || !all(map_lgl(columns, is.null))) {
       list(attributes = att, columns = columns)
     }
-  } else if (length(att)) {
-    list(attributes = att, columns = NULL)
+    return(out)
+  }
+
+  columns <- NULL
+  if (is.list(x) && !inherits(x, "POSIXlt")) {
+    # for list columns, we also keep attributes of each
+    # element in columns
+    columns <- map(x, arrow_attributes)
+    if (all(map_lgl(columns, is.null))) {
+      columns <- NULL
+    }
+  }
+
+  if (length(att) || !is.null(columns)) {
+    list(attributes = att, columns = columns)
   } else {
     NULL
   }
diff --git a/r/tests/testthat/test-metadata.R b/r/tests/testthat/test-metadata.R
index 1cd6fbc4599..e94c989ffae 100644
--- a/r/tests/testthat/test-metadata.R
+++ b/r/tests/testthat/test-metadata.R
@@ -134,3 +134,10 @@ test_that("metadata keeps attribute of top level data frame", {
   expect_identical(attr(as.data.frame(tab), "foo"), "bar")
   expect_identical(as.data.frame(tab), df)
 })
+
+test_that("metadata of list elements (ARROW-10386)", {
+  df <- data.frame(x = list(structure(1, foo = "bar"), structure(2, foo = "bar")))
+  tab <- Table$create(df)
+  expect_identical(attr(as.data.frame(tab)$x[[1]], "foo"), "bar")
+  expect_identical(attr(as.data.frame(tab)$x[[2]], "foo"), "bar")
+})

From 4d1e73d7c1f6fb8a3b5a51ce367d1f7de65bfd3c Mon Sep 17 00:00:00 2001
From: Romain Francois <romain@rstudio.com>
Date: Thu, 29 Oct 2020 12:25:54 +0100
Subject: [PATCH 2/2] update test

---
 r/tests/testthat/test-metadata.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/r/tests/testthat/test-metadata.R b/r/tests/testthat/test-metadata.R
index e94c989ffae..9b6dd0fddc6 100644
--- a/r/tests/testthat/test-metadata.R
+++ b/r/tests/testthat/test-metadata.R
@@ -136,7 +136,7 @@ test_that("metadata keeps attribute of top level data frame", {
 })
 
 test_that("metadata of list elements (ARROW-10386)", {
-  df <- data.frame(x = list(structure(1, foo = "bar"), structure(2, foo = "bar")))
+  df <- data.frame(x = I(list(structure(1, foo = "bar"), structure(2, foo = "bar"))))
   tab <- Table$create(df)
   expect_identical(attr(as.data.frame(tab)$x[[1]], "foo"), "bar")
   expect_identical(attr(as.data.frame(tab)$x[[2]], "foo"), "bar")