Ferry Generator Bug Report: Incorrect `.replace()` usage for primitive custom scalar types

[li-nkSN]

Hello, I employed claude to help me write this bug report. I'm willing to help support the resolution of this issue as well.

Ferry Generator Bug Report: Incorrect .replace() usage for primitive custom scalar types

Summary

When using type_overrides to map a custom GraphQL scalar to a primitive Dart type (e.g., DateTime), the ferry_generator incorrectly generates .replace() method calls in input type builders. Primitive types don't have a .replace() method - that's only for built_value types with builders.

Environment

• ferry: ^0.16.2-dev.4
• ferry_generator: ^0.14.0-dev.2
• built_value: ^8.9.3
• Dart SDK: 3.x

Configuration

# build.yaml
targets:
  $default:
    builders:
      ferry_generator|graphql_builder:
        enabled: true
        options:
          schema: li_nk_app|lib/graphql/schema.graphql
          type_overrides:
            OffsetDateTime:
              name: DateTime
              import: "dart:core"

      ferry_generator|serializer_builder:
        enabled: true
        options:
          schema: li_nk_app|lib/graphql/schema.graphql
          type_overrides:
            OffsetDateTime:
              name: DateTime
              import: "dart:core"
          custom_serializers:
            - import: "package:li_nk_app/graphql/serializers/offset_date_time_serializer.dart"
              name: OffsetDateTimeSerializer

Custom Serializer

class OffsetDateTimeSerializer implements PrimitiveSerializer<DateTime> {
  @override
  DateTime deserialize(Serializers serializers, Object serialized, {
    FullType specifiedType = FullType.unspecified,
  }) {
    return DateTime.parse(serialized as String);
  }

  @override
  Object serialize(Serializers serializers, DateTime dateTime, {
    FullType specifiedType = FullType.unspecified,
  }) =>
      dateTime.toUtc().toIso8601String();

  @override
  Iterable<Type> get types => [DateTime];

  @override
  String get wireName => 'OffsetDateTime';
}

GraphQL Schema (excerpt)

scalar OffsetDateTime

input CommentInput {
  content: String!
  createdAt: OffsetDateTime
  lastUpdated: OffsetDateTime
}

Incorrect Generated Code

In lib/graphql/__generated__/schema.schema.gql.dart:

// In the GCommentInputSerializer._fromMap() method:
case 'createdAt':
  var _$fieldValue = serializers.deserialize(value,
      specifiedType: const FullType(DateTime)) as DateTime;
  builder.createdAt.replace(_$fieldValue);  // ERROR!
  break;

Error Message

Error: The method 'replace' isn't defined for the type 'DateTime?'.
 - 'DateTime' is from 'dart:core'.
Try correcting the name to the name of an existing method, or defining a method named 'replace'.
          builder.createdAt.replace(_$fieldValue);
                            ^^^^^^^

Expected Generated Code

case 'createdAt':
  var _$fieldValue = serializers.deserialize(value,
      specifiedType: const FullType(DateTime)) as DateTime;
  builder.createdAt = _$fieldValue;  // Direct assignment for primitives
  break;

Root Cause Analysis

The generator appears to assume that any type mapped via type_overrides might be a BuiltValue type (which has builders and .replace() methods). However, when the target type is a primitive Dart type like DateTime, int, or String, the generator should use direct assignment (=) instead of .replace().

The .replace() method is only available on built_value builder fields for nested BuiltValue objects.

Workaround

Manually fix the generated code by replacing .replace() with direct assignment:

sed -i 's/builder\.createdAt\.replace(_\$fieldValue);/builder.createdAt = _\$fieldValue;/g' lib/graphql/__generated__/schema.schema.gql.dart
sed -i 's/builder\.lastUpdated\.replace(_\$fieldValue);/builder.lastUpdated = _\$fieldValue;/g' lib/graphql/__generated__/schema.schema.gql.dart

Affected Fields

Any nullable field in an input type that uses a custom scalar mapped to a primitive type:

• createdAt: OffsetDateTime → DateTime
• lastUpdated: OffsetDateTime → DateTime
• dateOfBirth: OffsetDateTime → DateTime

Suggested Fix

In the generator, when emitting builder assignment code for deserialized values, check if the target type is a primitive (or mapped to a primitive via type_overrides) and use direct assignment instead of .replace().

Related

• Ferry custom scalars documentation: https://ferrygraphql.com/docs/custom-scalars
• built_value serialization: https://medium.com/dartlang/darts-built-value-for-serialization-f5db9d0f4159