alamb commented on code in PR #9544:
URL: https://github.com/apache/arrow-rs/pull/9544#discussion_r2955915857
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -2276,6 +2309,137 @@ fn cast_struct_fields_in_order(
.collect::<Result<Vec<ArrayRef>, ArrowError>>()
}
+/// Cast a UnionArray to another UnionArray by casting each child array.
+fn cast_union_to_union(
+ array: &UnionArray,
+ from_fields: &UnionFields,
+ to_fields: &UnionFields,
+ _to_mode: UnionMode,
+ cast_options: &CastOptions,
+) -> Result<ArrayRef, ArrowError> {
+ let type_ids = array.type_ids().clone();
+ let offsets = array.offsets().cloned();
+
+ let new_children: Vec<ArrayRef> = from_fields
+ .iter()
+ .map(|(from_id, _from_field)| {
+ let (_, to_field) = to_fields
+ .iter()
+ .find(|(to_id, _)| *to_id == from_id)
+ .ok_or_else(|| {
+ ArrowError::CastError(format!(
+ "Cannot cast union: type_id {from_id} not found in
target union"
+ ))
+ })?;
+ let child = array.child(from_id);
+ cast_with_options(child.as_ref(), to_field.data_type(),
cast_options)
+ })
+ .collect::<Result<_, _>>()?;
+
+ let union = UnionArray::try_new(to_fields.clone(), type_ids, offsets,
new_children)?;
+ Ok(Arc::new(union))
+}
+
+/// Cast a UnionArray to a non-union type.
+///
+/// Finds the first variant whose type matches or can be cast to `to_type`,
+/// extracts values for rows where that variant is active (NULLing other rows),
+/// then casts the result to the target type. Prefers an exact type match over
+/// a cast-compatible one.
+///
+/// Since union extraction inherently introduces nulls for non-matching rows,
+/// the target type's inner fields are made nullable to avoid validation errors
+/// (e.g., casting to `List(non-nullable Utf8)` becomes `List(nullable Utf8)`).
+fn cast_union_to_type(
Review Comment:
Another potential option is to introduce the behavior into `union_extract`:
https://docs.rs/arrow/latest/arrow/compute/kernels/union_extract/fn.union_extract.html
For example, maybe `union_extract_with_option` that takes some sort of
options struct that allows better control over the extraction
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