[GitHub] [arrow-rs] nevi-me commented on a change in pull request #389: make slice work for nested types

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nevi-me commented on a change in pull request #389:
URL: https://github.com/apache/arrow-rs/pull/389#discussion_r667459042



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File path: arrow/src/array/array_struct.rs
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@@ -85,12 +85,7 @@ impl From<ArrayData> for StructArray {
     fn from(data: ArrayData) -> Self {
         let mut boxed_fields = vec![];
         for cd in data.child_data() {
-            let child_data = if data.offset() != 0 || data.len() != cd.len() {

Review comment:
       @jhorstmann @jorgecarleitao the problem was here. We take `data` which has children and their own offsets, then slice that data correctly to populate `boxed_fields`, but then we still use the `data` with the child arrays that aren't offset.
   
   The result's that the struct is correct when looking at it through `boxed_fields` (e.g. the print utility uses this), but once you need to do anything with `data`, it's as if the child values were never sliced.
   
   The lightbulb turned on while i was trying to figure out why a test for #491 was failing.
   
   There's a change that @bjchambers authored (https://github.com/apache/arrow-rs/commit/9f965612626cdf31187ce07ba5dbecc3503077b8) which addressed the child offsets, but my tests were failing because I needed to revert what they did.




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