Is it possible to generate generic classes?

[dlabs-matic-leva]

I'm using NestJs to generate OpenAPI specs and NestJS recommends using schema+references to express generic responses. Below is example OpenAPI specs that is generated following this advice:

openapi: 3.0.0
paths:
  /api/clients:
    get:
      operationId: clients.list
      summary: ''
      parameters: []
      responses:
        '200':
          description: ''
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/PaginatedResponse'
      tags:
        - Clients
      security:
        - oauth2: []
info:
  title: ...
  description: ...
  version: '1.0'
  contact: {}
tags: []
servers: []
components:
  securitySchemes:
    oauth2:
      type: oauth2
      flows:
        implicit:
          authorizationUrl: ...
          scopes:
            offline_access: offline_access
            openid: openid
            profile: profile
            email: email
  schemas:
    Client:
      type: object
      properties:
        id:
          type: string
        name:
          type: string
        surname:
          type: string
      required:
        - id
        - name
        - surname
    PaginatedResponse:
      type: object
      properties:
        page:
          type: number
        pageSize:
          type: number
        results:
          type: array
          items:
            $ref: '#/components/schemas/Client'
      required:
        - page
        - pageSize
        - results

This is Dart code that was generated from specs:

@RestApi()
abstract class ClientsClient {
  factory ClientsClient(Dio dio, {String? baseUrl}) = _ClientsClient;
  @GET('/api/clients')
  Future<Object> clientsList();
}
[...]
@JsonSerializable()
class PaginatedResponse {
  const PaginatedResponse({
    required this.page,
    required this.pageSize,
  });

  factory PaginatedResponse.fromJson(Map<String, Object?> json) => _$PaginatedResponseFromJson(json);

  final num page;
  final num pageSize;

  Map<String, Object?> toJson() => _$PaginatedResponseToJson(this);
}

Observe that PaginatedResponse.results is missing and return type of clientsList() is Future<Object>. I would expect to see code similar to this:

@RestApi()
abstract class ClientsClient {
  factory ClientsClient(Dio dio, {String? baseUrl}) = _ClientsClient;
  @GET('/api/clients')
  Future<PaginatedResponse<Client>> clientsList();
}
[...]
@JsonSerializable()
class PaginatedResponse<Data> {
  const PaginatedResponse({
    required this.page,
    required this.pageSize,
    required this.results,
  });

  factory PaginatedResponse.fromJson(Map<String, Object?> json) =>
      _$PaginatedResponseFromJson(json);

  final num page;
  final num pageSize;
  final Data results;

  Map<String, Object?> toJson() => _$PaginatedResponseToJson(this);
}

Is this possible to achieve?

[dlabs-matic-leva]

Current workaround I'm doing is to create separate class for each paginated endpoint (PaginatedResponseForClients, PaginatedResponseForAccounts) but it would be nice to use generics.

[StarProxima]

OpenAPI does not directly support generics for classes. The implementation would be quite problematic and could not be universal for every specification.

There is not enough data in your NestJS-generated specification to realize that PaginatedResponse should be a generic class.

Usually, for such purposes, the spec automatically creates separate classes for each type as you described. This is what FastAPI does, for example.