[BUG][Typescript] Generated code with oneOf does not properly check the dto type

[artemka-debug]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Given the OpenApi schema below, we have Dto with payload field, which has a type of oneOf of Payload1, Payload2 and Payload3. Each of payload types has a property type with type of enum with only one value.

When we generate client code, we get this code in models/DtoPayload.ts file:

export function DtoPayloadToJSON(value?: DtoPayload | null): any {
    if (value == null) {
        return value;
    }

    if (instanceOfPayload1(value)) {
        return Payload1ToJSON(value as Payload1);
    }
    if (instanceOfPayload2(value)) {
        return Payload2ToJSON(value as Payload2);
    }
    if (instanceOfPayload3(value)) {
        return Payload3ToJSON(value as Payload3);
    }

    return {};
}

and code for each if check:

models/Payload1.ts

export function instanceOfPayload1(value: object): boolean {
    if (!('type' in value)) return false;
    if (!('barcode' in value)) return false;
    return true;
}

models/Payload2.ts

export function instanceOfPayload2(value: object): boolean {
    if (!('type' in value)) return false;
    return true;
}

models/Payload3.ts

export function instanceOfPayload3(value: object): boolean {
    if (!('type' in value)) return false;
    if (!('barcode' in value)) return false;
    return true;
}

since functions instanceOfPayload1 and instanceOfPayload3 are identical, generated code treats Payload3 as Payload1, which causes problems when converting plain objects to dto's.

openapi-generator version

7.5.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.0",
  "paths": {
    "/endpoint": {
      "get": {
        "operationId": "endpoint",
        "summary": "endpoint",
        "responses": {
          "200": {
            "description": "",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Dto"
                }
              }
            }
          }
        },
        "security": [
          {
            "access-token": []
          }
        ]
      }
    }
  },
  "info": {
    "title": "API",
    "description": "API",
    "version": "1.0",
    "contact": {},
    "license": {
      "name": "UNLICENSED",
      "url": "UNLICENSED"
    }
  },
  "tags": [],
  "servers": [
  ],
  "components": {
    "securitySchemes": {
      "access-token": {
        "scheme": "bearer",
        "bearerFormat": "JWT",
        "type": "http",
        "description": "JWT Authorization header using the Bearer scheme."
      }
    },
    "schemas": {
      "Payload3": {
        "type": "object",
        "properties": {
          "type": {
            "type": "string",
            "enum": [
              "AGE_VERIFICATION"
            ]
          },
          "barcode": {
            "type": "string"
          },
          "requiredAge": {
            "type": "number"
          }
        },
        "required": [
          "type",
          "barcode"
        ]
      },
      "Payload2": {
        "type": "object",
        "properties": {
          "type": {
            "type": "string",
            "enum": [
              "ASSISTANCE_REQUESTED"
            ]
          }
        },
        "required": [
          "type"
        ]
      },
      "Payload1": {
        "type": "object",
        "properties": {
          "type": {
            "type": "string",
            "enum": [
              "PARTIAL_RESCAN"
            ]
          },
          "barcode": {
            "type": "string"
          }
        },
        "required": [
          "type",
          "barcode"
        ]
      },
      "Dto": {
        "type": "object",
        "properties": {
          "payload": {
            "description": "Payload for the assistance request",
            "oneOf": [
              {
                "$ref": "#/components/schemas/Payload1"
              },
              {
                "$ref": "#/components/schemas/Payload2"
              },
              {
                "$ref": "#/components/schemas/Payload3"
              }
            ]
          }
        },
        "required": [
          "payload"
        ]
      }
    }
  }
}

Generation Details

npx -y @openapitools/openapi-generator-cli generate -g typescript-fetch
--additional-properties=importFileExtension=".js" --skip-validate-spec
-i ./test.json
-o src/api-client

Steps to reproduce

1. make directory: test
2. cd into directory test
3. create file test.json and copy and paste provided api schema above.
4. run provided in Generation Details command in cli

Related issues/PRs

Suggest a fix

We want the generator to also take into account types of fields, when checking if object is instance of another object.

for example:

export function instanceOfPayload1(value: object): boolean {
    if (!('type' in value && value.type === Object.values(Payload1TypeEnum)[0])) return false;
    if (!('barcode' in value)) return false;
    return true;
}

[odiak]

How about using discriminator ?
Specifying it will change generated code.

Like:

Dto:
  type: object
  properties:
    payload:
      oneOf:
        - $ref: '#/components/schemas/Payload1'
        - $ref: '#/components/schemas/Payload2'
        - $ref: '#/components/schemas/Payload3'
      discriminator:
        propertyName: type

[AnotiaWang]

How about using discriminator ? Specifying it will change generated code.

This does not work. With the provided schema in this issue, using discriminator will generate the following code:

/**
 * @type DtoPayload
 * Payload for the assistance request
 * @export
 */
export type DtoPayload = ;

And if I set "legacyDiscriminatorBehavior": false in openapitools.json, it will generate the following code:

/**
 * @type DtoPayload
 * Payload for the assistance request
 * @export
 */
export type DtoPayload = { type: 'Payload1' } & Payload1 | { type: 'Payload2' } & Payload2 | { type: 'Payload3' } & Payload3;
//          ^^^^^^^^^^ The type will be `never`

export function DtoPayloadFromJSON(json: any): DtoPayload {
    return DtoPayloadFromJSONTyped(json, false);
}

export function DtoPayloadFromJSONTyped(json: any, ignoreDiscriminator: boolean): DtoPayload {
    if (json == null) {
        return json;
    }
    switch (json['type']) {
        case 'Payload1':
            return Object.assign({}, Payload1FromJSONTyped(json, true), { type: 'Payload1' });
        //  ^^^^^^ Cannot assign type "Payload1 & { type: string; }" to type "never".
        case 'Payload2':
            return Object.assign({}, Payload2FromJSONTyped(json, true), { type: 'Payload2' });
        case 'Payload3':
            return Object.assign({}, Payload3FromJSONTyped(json, true), { type: 'Payload3' });
        default:
            throw new Error(`No variant of DtoPayload exists with 'type=${json['type']}'`);
    }
}

[odiak]

Hmm, there's room for improvement in the generated code.

[AnotiaWang]

Maybe we can fix it like this:

openapi-generator/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

Line 26 in 1c787ba

if (!('{{name}}' in value) || value['{{name}}'] === undefined) return false;

-     if (!('{{name}}' in value){{#isEnum}} || value['{{name}}'] !== {{enumType}}[0]{{/isEnum}}) return false;
+     if (!('{{name}}' in value){{#isEnum}} || value['{{name}}'] !== Object.values({{datatypeWithEnum}})[0]{{/isEnum}}) return false;