Fail to deserialize generated unions

[oringnes]

Hi,

When working with a union of records, I am unable to deserialize record using apache_avro.
Example schema Foo.avsc:

{
  "type": "record",
  "name": "Foo",
  "fields": [
    {"name": "a", "type": "int"},
    {"name": "b",
      "type":
      [
        {
          "type": "record",
          "name": "Bar",
          "fields": [
            {"name": "c", "type": "long"}
          ]
        },
        {
          "type": "record",
          "name": "Baz",
          "fields": [
            {"name": "d", "type": "int"}
          ]
        }
      ]
    }
  ]
}

Expecting struct to be identical when serialized, and then deserialized:

    #[test]
    fn test_foo_serde() {
        let before = Foo {
            a: 0,
            b: UnionBarBaz::Baz(Baz {
                d: 42,
            })
        };

        let schema = Schema::parse_str(include_str!("../avsc/Foo.avsc")).unwrap();

        let mut writer = Writer::new(&schema, Vec::new());
        writer.append_ser(before.clone()).unwrap();
        let encoded = writer.into_inner().unwrap();

        let mut reader = Reader::with_schema(&schema, &encoded[..]).unwrap();
        let value = reader.next().unwrap();
        let after: Foo = from_value::<Foo>(&value.unwrap()).unwrap();

        assert_eq!(before, after);
    }

The call to from_value() is responding with the following error:

Failed to deserialize Avro value into value: Expected a Record|Enum, but got Union(1, Record([("d", Int(42))]))

[lerouxrgd]

Hi,

This is actually a tricky one. The root of this problem stems from the mismatch between the "Union variant" that gets serialized and the "Record value" that is expected by the deserializer.

Handling this for basic types (ints, floats, etc) isn't too hard and is already implemented, it is available with the flag --union-deser. However for potentially recursive types, such as a record, I haven't found a good solution yet as things can get pretty nasty to template/generate. In the code right now, when using --union-deser GenUnionVisitor::serde_visitor will be set to None, which, when templated, will be discarded.

Any help on this topic is welcomed.

[lerouxrgd]

It might also be possible to handle this issue directly in apache_avro, I would to look into it.