From 53e89c87c3e28a99335318a922d5afcd0278b664 Mon Sep 17 00:00:00 2001
From: Etienne Bacher <52219252+etiennebacher@users.noreply.github.com>
Date: Thu, 8 Aug 2024 14:13:27 +0200
Subject: [PATCH] feat: `as_polars_df()` keeps the schema when converting a
 `nanoarrow_array` with 0 rows

---
 NEWS.md                         |  2 ++
 R/as_polars.R                   |  8 +-------
 tests/testthat/test-as_polars.R | 11 +++++++++++
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index fc887bd8e..fa58664bf 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -5,6 +5,8 @@
 ### New features
 
 - New method `$str$extract_many()` (#1163).
+- Converting a `nanoarrow_array` with zero rows to an `RPolarsDataFrame` via
+  `as_polars_df()` now keeps the original schema (#1177).
 
 ### Bug fixes
 
diff --git a/R/as_polars.R b/R/as_polars.R
index a75232b2a..4d7279433 100644
--- a/R/as_polars.R
+++ b/R/as_polars.R
@@ -257,13 +257,7 @@ as_polars_df.nanoarrow_array = function(x, ...) {
   }
 
   series = as_polars_series.nanoarrow_array(x, name = NULL)
-
-  if (length(series)) {
-    series$to_frame()$unnest("")
-  } else {
-    # TODO: support 0-length array
-    pl$DataFrame()
-  }
+  series$to_frame()$unnest("")
 }
 
 
diff --git a/tests/testthat/test-as_polars.R b/tests/testthat/test-as_polars.R
index 3c04f46a5..c84c65311 100644
--- a/tests/testthat/test-as_polars.R
+++ b/tests/testthat/test-as_polars.R
@@ -513,3 +513,14 @@ patrick::with_parameters_test_that(
   },
   .cases = make_as_polars_df_experimental_cases()
 )
+
+
+test_that("as_polars_df works for nanoarrow_array with zero rows", {
+  skip_if_not_installed("nanoarrow")
+  orig = data.frame(col1 = character(0), col2 = numeric(0))
+  out = nanoarrow::as_nanoarrow_array(orig) |>
+    as_polars_df() |>
+    as.data.frame()
+  expect_identical(out, orig)
+  expect_identical(lapply(out, class), list(col1 = "character", col2 = "numeric"))
+})