Discriminator in Open API JSON Causes "Type expected" Error w/RTK Query Code Gen

[fuzl-llc]

I have an API and use NSwag.AspNetCore to generate open api JSON. I have a "WidgetBase" abstract type with several derived types that are distinquished by a WidgetType property. The generated JSON used to be this:

      "WidgetBase": {
        "type": "object",
        "x-abstract": true,
        "additionalProperties": false,
        "properties": {
          "widgetType": {
            "type": "string"
          }
        }
      }

With the above, RTK Query Code Gen runs perfectly fine.

I updated to a newer version of NSwag and it now adds some additional info (discriminator, required/$type, and properties/$type):

      "WidgetBase": {
          "type": "object",
          "discriminator": {
              "propertyName": "$type"
          },
          "x-abstract": true,
          "additionalProperties": false,
          "required": [
              "$type"
          ],
          "properties": {
              "widgetType": {
                  "type": "string"
              },
              "$type": {
                  "type": "string"
              }
          }
      }

Now when I run RTK Query Code Gen, I get this error:

Generating ./fuzlApiTypes.ts
SyntaxError: Type expected. (152:26)
�[0m �[90m 150 |�[39m     errorMessage�[33m?�[39m�[33m:�[39m string�[33m;�[39m
 �[90m 151 |�[39m }�[33m;�[39m
�[31m�[1m>�[22m�[39m�[90m 152 |�[39m �[36mexport�[39m type �[33mWidgetBase�[39m �[33m=�[39m �[33m;�[39m
 �[90m     |�[39m                          �[31m�[1m^�[22m�[39m
 �[90m 153 |�[39m �[36mexport�[39m type �[33mWidgetDrawer�[39m �[33m=�[39m {
 �[90m 154 |�[39m     widgets�[33m?�[39m�[33m:�[39m �[33mWidgetBase�[39m[]�[33m;�[39m
 �[90m 155 |�[39m }�[33m;�[39m�[0m
    at _4 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:17:70560)
    at N4 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:20:955)
    at Object.M4 [as parse] (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:20:1477)
    at parse5 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/index.mjs:19838:24)
    at coreFormat (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/index.mjs:20386:25)
    at formatWithCursor (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/index.mjs:20598:14)
    at Object.format2 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/index.mjs:22002:25)
    at generateEndpoints (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/@rtk-query/codegen-openapi/src/index.ts:25:7)
    at v (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/@rtk-query/codegen-openapi/src/bin/cli.ts:59:7) {
  loc: { start: { line: 152, column: 26 }, end: { line: 152, column: 26 } },
  cause: $f: Type expected.
      at ed (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:17:8588)
      at ad (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:17:10524)
      at Xh (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:17:65505)
      at a4 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:17:70421)
      at l0 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:17:70163)
      at file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:20:1441
      at s4 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:17:70714)
      at Object.M4 [as parse] (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/plugins/typescript.mjs:20:1425)
      at parse5 (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/index.mjs:19838:24)
      at coreFormat (file:///C:/Dev/Code/Fuzl/Apps/client/node_modules/prettier/index.mjs:20386:25) {
    fileName: 'estree.ts',
    location: { end: [Object], start: [Object] }
  },
  codeFrame: '\x1B[0m \x1B[90m 150 |\x1B[39m     errorMessage\x1B[33m?\x1B[39m\x1B[33m:\x1B[39m string\x1B[33m;\x1B[39m\n' +
    ' \x1B[90m 151 |\x1B[39m }\x1B[33m;\x1B[39m\n' +
    '\x1B[31m\x1B[1m>\x1B[22m\x1B[39m\x1B[90m 152 |\x1B[39m \x1B[36mexport\x1B[39m type \x1B[33mWidgetBase\x1B[39m \x1B[33m=\x1B[39m \x1B[33m;\x1B[39m\n' +
    ' \x1B[90m     |\x1B[39m                          \x1B[31m\x1B[1m^\x1B[22m\x1B[39m\n' +
    ' \x1B[90m 153 |\x1B[39m \x1B[36mexport\x1B[39m type \x1B[33mWidgetDrawer\x1B[39m \x1B[33m=\x1B[39m {\n' +
    ' \x1B[90m 154 |\x1B[39m     widgets\x1B[33m?\x1B[39m\x1B[33m:\x1B[39m \x1B[33mWidgetBase\x1B[39m[]\x1B[33m;\x1B[39m\n' +
    ' \x1B[90m 155 |\x1B[39m }\x1B[33m;\x1B[39m\x1B[0m'
}

I can't make much sense of this but see "SyntaxError: Type expected" and "WidgetBase" mentioned. The JSON seems to indicate that I have a "type" property on WidgetBase which I do not so I tried to make discriminator/propertyName referernce the widgetType property by adding JsonPolymorphicAttribute to my WidgetBase type in the C# code:

    [JsonPolymorphic(TypeDiscriminatorPropertyName = "widgetType")]
    [JsonDerivedType(typeof(Widget2))]
    [JsonDerivedType(typeof(Widget2))]
    public abstract class WidgetBase
    {
        public string WidgetType { get; set; } = String.Empty;
    }

That then resulted in this, seemingly better, JSON that correctly says that "widgetType" is the thing that can be used to discriminate between derived types:

      "WidgetBase": {
        "type": "object",
        "discriminator": {
          "propertyName": "widgetType"
        },
        "x-abstract": true,
        "additionalProperties": false,
        "required": [
          "widgetType"
        ],
        "properties": {
          "widgetType": {
            "type": "string"
          }
        }
      }

This seemed better since the WidgetBase type does have a WidgetType property. That property is Pascal case in C# and camel case in the generated client code and I tried both for the JsonPolymorphicAttribute just in case but either way I get basically the same error when I try to generate code with the RTK Query Code Gen.

Does RTK Query Code Gen support this discriminator value? I could be doing something wrong, but it seems to break when nothing really changes except adding the extra discriminator and related JSON to the API spec.

[fuzl-llc]

I have it working now. I don't fully understand it but thought I'd post in case it helps someone and/or there is anything to be done on the code gen part. I'm guessing not since it seems to do with how I was using NJsonSchema and the resulting Open API JSON document's format... though perhaps the error above could be made to be more descriptive - it didn't really help me much but maybe that is my lack of understanding...

Anyway, I'd put the [JsonSchemaFlatten] on my derived types a long time ago for some reason. I remove those and adjusted the way I was using the [JsonDerivedType] attribute on the base class. I also removed the [TypeDiscriminatorPropertyName] attribute to accept the default "$type" value:

[JsonDerivedType(typeof(Widget1), "Widget1")]
[JsonDerivedType(typeof(Widget2), "Widget2")]

These changes resulted in a more expected Open API JSON output which included references to derived types in a mapping from the discriminator value to the type:

      "WidgetBase": {
        "type": "object",
        "discriminator": {
          "propertyName": "$type",
          "mapping": {
            "Widget1": "#/components/schemas/Widget1",
            "Widget2": "#/components/schemas/Widget2",
          }
        },
        "x-abstract": true,
        "additionalProperties": false,
        "required": [
          "$type"
        ],
        "properties": {
          "widgetType": {
            "type": "string"
          },
          "$type": {
            "type": "string"
          }
        }
      }

...as well as the allOf construct to include base type properties in the derived types:

      "Widget1": {
        "allOf": [
          {
            "$ref": "#/components/schemas/WidgetBase"
          },
          {
            "type": "object",
            "additionalProperties": false,
            "properties": {
              "body": {
                "type": "string"
              }
            }
          }
        ]
      }

Then the RTK Query Open API gen worked just fine.