Discriminated subclasses fields doesn't have MappableField for correct key name.

[danialb007]

Steps to reproduce

1. Use DartMappable -> json_serializer: dart_mappable
2. Have some schema with oneOf, discriminator setup, but make sure fields have snake case like this -> is_active
3. Start the generation and use the classes. you will have an error like this ->
   [ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: MapperException: Failed to decode (BaseType).encounters(List<SubClassTypeSealed>).item(SubClassTypeSealed).isActive: Parameter isActive is missing.

Fix Suggestion

Normal classes and types (Not discriminated) are fine and don't have this problem.
Adding this @MappableField(key: 'is_active') above the fields in the mappable classes (annotated with @MappableClass()) before code generation would fix it.

I think this code in the file lib/src/generator/templates/dart_dart_mappable_dto_template.dart:200 is related.

/// if jsonKey is different from the name
String _jsonKey(UniversalType t) {
  if (t.jsonKey == null || t.name == t.jsonKey) {
    return '';
  }
  return "${indentation(2)}@MappableField(key: '${protectJsonKey(t.jsonKey)}')\n";
}

Also changing line lib/src/generator/templates/dart_dart_mappable_dto_template.dart:691 and adding '_jsonKey' fixes this, but not sure if this is the best fix.

'${_jsonKey(prop)}${indentation(2)}@override\n${indentation(2)}final ${_renameUnionTypes(prop.toSuitableType(ProgrammingLanguage.dart, useMultipartFile: useMultipartFile))} ${prop.name};',

Expected results

No errors occurring.

Actual results

Mentioned in the steps to reproduce section.

Your OpenApi snippet

Here is an array that has different types inside.

"different_types_array": {
    "title": "DifferentTypesArray",
    "type": "array"
    "items": {
        "discriminator": {
            "mapping": {
                "a": "#/components/schemas/SubSchemaA",
                "b": "#/components/schemas/SubSchemaB",
                "c": "#/components/schemas/SubSchemaC"
            },
            "propertyName": "resource_type"
        },
        "oneOf": [
            {
                "$ref": "#/components/schemas/SubSchemaA"
            },
            {
                "$ref": "#/components/schemas/SubSchemaB"
            },
            {
                "$ref": "#/components/schemas/SubSchemaC"
            }
        ]
    },
},

Code sample

Code sample

switch (element) {
  SubClassA a => a,
  SubClassB b => b,
  SubClassC c => c,
},

Logs

Logs

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: MapperException: Failed to decode (BaseType).encounters(List<SubClassTypeSealed>).item(SubClassTypeSealed).isActive: Parameter isActive is missing.

Dart version and used packages versions

Dart version

Dart SDK version: 3.10.4 (stable) (Tue Dec 9 00:01:55 2025 -0800) on "macos_arm64"

Packages version

swagger_parser: ^1.42.0