V0ldek opened a new issue, #6349:
URL: https://github.com/apache/arrow-rs/issues/6349
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
I'm trying to use `StructBuilder` to create a struct whose one field is of a
Union type. This is currently unsupported, see this small repro (crate version
v52.2.0):
```rust
use arrow::{
array::StructBuilder,
datatypes::{DataType, Field, Fields, UnionFields, UnionMode},
};
fn main() {
let _builder = StructBuilder::from_fields(
get_struct_fields(),
3,
);
}
fn get_struct_fields() -> Fields {
use arrow::datatypes::{DataType, Field, Fields};
Fields::from(vec![
Field::new("name", DataType::Utf8, false),
Field::new("value", get_union_type(), false),
])
}
fn get_union_type() -> DataType {
let fields = vec![
Field::new("Integer", DataType::Int32, false),
Field::new("Float", DataType::Float32, false),
];
DataType::Union(UnionFields::new(vec![0, 1], fields), UnionMode::Sparse)
}
```
This results in a panic from `builder/struct_builder.rs:278:14`:
```
Data type Union([(0, Field { name: "Integer", data_type: Int32, nullable:
false, dict_id: 0, dict_is_ordered: false, metadata: {} }), (1, Field { name:
"Float", data_type: Float32, nullable: false, dict_id: 0, dict_is_ordered:
false, metadata: {} })], Sparse) is not currently supported
```
**Describe the solution you'd like**
I'd like to use `StructBuilder` with a Union field the same way it works
with other field types. In particular, I should be able to retrieve a
`UnionBuilder` for a field:
```
let mut builder = StructBuilder::from_fields(
get_struct_fields(),
3,
);
let union_field = builder.field_builder::<UnionBuilder>(1);
```
**Describe alternatives you've considered**
I don't actually know how to work around this. It'd be appreciated if
someone could show me how to build a struct array with a schema like this
without using the builder, so I can unblock myself.
**Additional context**
As far as I understand the main issue is that `UnionBuilder` does not
implement the `ArrayBuilder` trait, which is required for the dynamic API and
internals of `StructBuilder`. This is because `ArrayBuilder` requires a
`finish(&mut self) -> ArrayRef`, and the `UnionBuilder` has a `build(self) ->
Result<UnionArray, ArrowError>` (consumes the builder AND can fail).
Changing `self` to `&mut self` shouldn't be too hard, but the `Result`
semantics are rather hard. I'm not sure what the implementation path would be
here.
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