Mark discriminator properties consistently as requiered

.chronus/changes/fix-discriminator-should-be-requiered-2024-9-9-17-24-19.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Introducing a fix so that all discriminator properties are marked as required.

packages/openapi3/src/schema-emitter.ts

@@ -295,6 +295,13 @@ export class OpenAPI3SchemaEmitter extends TypeEmitter<
       }
     }
 
+    const discriminator = getDiscriminator(this.emitter.getProgram(), model);
+    if (discriminator) {
+      if (!requiredProps.includes(discriminator.propertyName)) {
+        requiredProps.push(discriminator.propertyName);
+      }
+    }
+
     return requiredProps.length > 0 ? requiredProps : undefined;
   }
 

packages/openapi3/test/discriminator.test.ts

@@ -103,7 +103,7 @@ describe("openapi3: polymorphic model inheritance with discriminator", () => {
         name: { type: "string" },
         weight: { type: "number", format: "float" },
       },
-      required: ["name"],
+      required: ["name", "kind"],
       discriminator: {
         propertyName: "kind",
         mapping: {
@@ -154,7 +154,7 @@ describe("openapi3: polymorphic model inheritance with discriminator", () => {
         },
         weight: { type: "number", format: "float" },
       },
-      required: ["name"],
+      required: ["name", "kind"],
       discriminator: {
         propertyName: "kind",
         mapping: {
@@ -215,7 +215,7 @@ describe("openapi3: polymorphic model inheritance with discriminator", () => {
         name: { type: "string" },
         weight: { type: "number", format: "float" },
       },
-      required: ["name"],
+      required: ["name", "kind"],
       discriminator: {
         propertyName: "kind",
         mapping: {
@@ -234,7 +234,7 @@ describe("openapi3: polymorphic model inheritance with discriminator", () => {
         },
         bark: { type: "string" },
       },
-      required: ["kind", "bark"],
+      required: ["kind", "bark", "breed"],
       allOf: [{ $ref: "#/components/schemas/Pet" }],
       discriminator: {
         propertyName: "breed",
@@ -344,4 +344,16 @@ describe("openapi3: polymorphic model inheritance with discriminator", () => {
       },
     ]);
   });
+
+  it("discriminator always needs to be marked as required", async () => {
+    const openApi = await openApiFor(`
+     @discriminator("kind")
+     model Animal {
+     id: string;}`);
+
+    deepStrictEqual(openApi.components.schemas.Animal.required, ["id", "kind"]);
+    deepStrictEqual(openApi.components.schemas.Animal.discriminator, {
+      propertyName: "kind",
+    });
+  });
 });