nevi-me opened a new issue #825:
URL: https://github.com/apache/arrow-rs/issues/825
**Describe the bug**
If a list of a nested struct is empty, the JSON reader should not create a
null struct value.
**To Reproduce**
Try to write an array of below:
```rust
#[test]
fn json_list_roundtrip() {
let json_content = r#"
{"list": [{"ints": 1}]}
{"list": [{}]}
{"list": []}
{"list": null}
{"list": [{"ints": null}]}
{"list": [null]}
"#;
let ints_struct =
DataType::Struct(vec![Field::new("ints", DataType::Int32, true)]);
let list_type = DataType::List(Box::new(Field::new("item", ints_struct,
true)));
let list_field = Field::new("list", list_type, true);
let schema = Arc::new(Schema::new(vec![list_field]));
let builder =
ReaderBuilder::new().with_schema(schema).with_batch_size(64);
let mut reader =
builder.build(std::io::Cursor::new(json_content)).unwrap();
let batch = reader.next().unwrap().unwrap();
let list_row = batch
.column(0)
.as_any()
.downcast_ref::<ListArray>()
.unwrap();
let values = list_row.values();
// the {"list": []} value gets interpreted as an empty struct, causing
the length below to be 5
assert_eq!(values.len(), 4);
assert_eq!(values.null_count(), 1);
}
```
**Expected behavior**
Empty lists should not create an empty/null struct as this can lead to
incorrect writing of data as the list's child length would disagree with the
offsets.
**Additional context**
This relates to #704 but doesn't yet fix it
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