Polymorphic derived types do not have discriminator property marked as required in OpenAPI schema.

[andrewimcclement]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

[JsonDerivedType(typeof(Bar), "bar")]
[JsonDerivedType(typeof(Baz), "baz")]
public abstract record Foo;
public record Bar(int Value) : Foo;
public record Baz(string Name) : Foo;

Generated schema

  "components": {
    "schemas": {
      "Foo": {
        "required": [
          "$type"
        ],
        "type": "object",
        "anyOf": [
          {
            "$ref": "#/components/schemas/FooBar"
          },
          {
            "$ref": "#/components/schemas/FooBaz"
          }
        ],
        "discriminator": {
          "propertyName": "$type",
          "mapping": {
            "bar": "#/components/schemas/FooBar",
            "baz": "#/components/schemas/FooBaz"
          }
        }
      },
      "FooBar": {
        "required": [
          "value"
        ],
        "properties": {
          "$type": {
            "enum": [
              "bar"
            ],
            "type": "string"
          },
          "value": {
            "type": "integer",
            "format": "int32"
          }
        }
      },
      "FooBaz": {
        "required": [
          "name"
        ],
        "properties": {
          "$type": {
            "enum": [
              "baz"
            ],
            "type": "string"
          },
          "name": {
            "type": "string"
          }
        }
      }
    }
  }

In the schema, FooBar and FooBaz have required properties, but critically the discriminator property $type is not required,
even though it is required in the base class Foo. This is inconsistent and can cause issues when e.g.
trying to generate TypeScript classes from the Open API schema.

Expected Behavior

Expected generated schema

  "components": {
    "schemas": {
      "Foo": {
        "required": [
          "$type"
        ],
        "type": "object",
        "anyOf": [
          {
            "$ref": "#/components/schemas/FooBar"
          },
          {
            "$ref": "#/components/schemas/FooBaz"
          }
        ],
        "discriminator": {
          "propertyName": "$type",
          "mapping": {
            "bar": "#/components/schemas/FooBar",
            "baz": "#/components/schemas/FooBaz"
          }
        }
      },
      "FooBar": {
        "required": [
          "$type",
          "value"
        ],
        "properties": {
          "$type": {
            "enum": [
              "bar"
            ],
            "type": "string"
          },
          "value": {
            "type": "integer",
            "format": "int32"
          }
        }
      },
      "FooBaz": {
        "required": [
          "$type",
          "name"
        ],
        "properties": {
          "$type": {
            "enum": [
              "baz"
            ],
            "type": "string"
          },
          "name": {
            "type": "string"
          }
        }
      }
    }
  }

Steps To Reproduce

An example reproducing this issue can be found at https://github.com/andrewimcclement/openApiDiscriminatorIssue.

Exceptions (if any)

No response

.NET Version

9.0.2

Anything else?

ASP.NET Core version: 9.0.2.

I do not believe this is the same as #57982

[mikekistler]

Thank you for filing this issue. I can see how some downstream tools might work better if the discriminator of derived types was marked as required. But OpenAPI only requires that the discriminator be required in the base schema, so I would not say the current behavior is a bug. I'm going to mark this as a feature and move it to our backlog and we might be able to squeeze it into the next release at some point.