エラー内容

Missing concrete implementations of '_$Post.toJson', 'getter _$Post.body', 'getter _$Post.createdAt', 'getter _$Post.imageUrl', and 3 more.
Try implementing the missing methods, or make the class abstract.

エラーが出たファイル

/// post.dart
import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:riverpod_training/function/timestamp_converter.dart';

part 'post.freezed.dart';
part 'post.g.dart';

@freezed
class Post with _$Post {
  factory Post({
    required String body,
    required String userId,
    required String postId,
    required String imageUrl,
    @TimestampConverter() required Timestamp createdAt,
    @TimestampConverter() required Timestamp updatedAt,
  }) = _Post;

  factory Post.fromJson(Map<String, dynamic> json) => _$PostFromJson(json);
}

エラー再現

post.dartを作成して、ターミナルで以下のコードを叩く

flutter pub run build_runner build --delete-conflicting-outputs

その結果、post.freezed.dartとpost.g.dartが作成されるが、先ほどのエラーが発生する。

解決方法

post.freezed.dartを以下のように書き加える

...
// dart format off
T _$identity<T>(T value) => value;

+ final _privateConstructorUsedError = UnsupportedError(
+   'It seems like you constructed your class using `MyClass._()`. This constructor is only meant to be used by freezed and you are not supposed to need it nor use it.\nPlease check the documentation here for more information: https://github.com/rrousselGit/freezed#adding-getters-and-methods-to-our-models',
+ );

/// @nodoc
mixin _$Post {

- String get body; String get userId; String get postId; String get imageUrl;@TimestampConverter() Timestamp get createdAt;@TimestampConverter() Timestamp get updatedAt;
+ String get body => throw _privateConstructorUsedError; String get userId => throw _privateConstructorUsedError; String get postId => throw _privateConstructorUsedError; String get imageUrl => throw _privateConstructorUsedError;@TimestampConverter() Timestamp get createdAt => throw _privateConstructorUsedError;@TimestampConverter() Timestamp get updatedAt => throw _privateConstructorUsedError;
/// Create a copy of Post
/// with the given fields replaced by the non-null parameter values.
@JsonKey(includeFromJson: false, includeToJson: false)
@pragma('vm:prefer-inline')
$PostCopyWith<Post> get copyWith => _$PostCopyWithImpl<Post>(this as Post, _$identity);

  /// Serializes this Post to a JSON map.
-    Map<String, dynamic> toJson();
+    Map<String, dynamic> toJson() => throw _privateConstructorUsedError;