JSON.mapping宏如何处理联合类型的参数？

require "json"

class A
  JSON.mapping(
    field1: String,
    field2: Int32
  )
end

class B
  JSON.mapping(
    field3: Float64,
    field4: Bool
  )
end

class UnionType
  JSON.mapping(
    case field
    when A
      field_a : A
    when B
      field_b : B
    end
  )
end

data = %({"field_a": {"field1": "value1", "field2": 42}})

union_type = JSON.parse(UnionType, data)
case union_type.field
when A
  puts union_type.field_a.field1
when B
  puts union_type.field_b.field3
end

json_data = JSON.generate(union_type)
puts json_data