feat: as_polars_df() keeps the schema when converting a `nanoarrow_… (

#1177)

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
 

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("")
 }
 
 

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"))
+})