[GitHub] [arrow-rs] tustvold commented on a diff in pull request #1794: Write validity buffer for UnionArray in V4 IPC message

[GitBox <gi...@apache.org>]

tustvold commented on code in PR #1794:
URL: https://github.com/apache/arrow-rs/pull/1794#discussion_r889964435


##########
arrow/src/ipc/writer.rs:
##########
@@ -1433,4 +1443,68 @@ mod tests {
             },
         );
     }
+
+    fn write_union_file(options: IpcWriteOptions) {
+        let schema = Schema::new(vec![Field::new(
+            "union",
+            DataType::Union(
+                vec![
+                    Field::new("a", DataType::Int32, false),
+                    Field::new("c", DataType::Float64, false),
+                ],
+                vec![0, 1],
+                UnionMode::Sparse,
+            ),
+            true,
+        )]);
+        let mut builder = UnionBuilder::new_sparse(5);
+        builder.append::<Int32Type>("a", 1).unwrap();
+        builder.append_null::<Int32Type>("a").unwrap();
+        builder.append::<Float64Type>("c", 3.0).unwrap();
+        builder.append_null::<Float64Type>("c").unwrap();
+        builder.append::<Int32Type>("a", 4).unwrap();
+        let union = builder.build().unwrap();
+
+        let batch = RecordBatch::try_new(
+            Arc::new(schema.clone()),
+            vec![Arc::new(union) as ArrayRef],
+        )
+        .unwrap();
+        let file_name = "target/debug/testdata/union.arrow_file";
+        {
+            let file = File::create(&file_name).unwrap();
+            let mut writer =
+                FileWriter::try_new_with_options(file, &schema, options).unwrap();
+
+            writer.write(&batch).unwrap();
+            writer.finish().unwrap();
+        }
+
+        {
+            let file = File::open(&file_name).unwrap();
+            let reader = FileReader::try_new(file, None).unwrap();

Review Comment:
   Not sure how hard this would be to test, but perhaps we should test what null mask is actually written (I think the reader simply ignores it)



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