From de763ceb0f69c994006fef74273b1bdea55e05f9 Mon Sep 17 00:00:00 2001
From: Alitzel Mendez <6895254+AlitzelMendez@users.noreply.github.com>
Date: Mon, 14 Oct 2024 14:12:29 -0700
Subject: [PATCH] Mark discriminator properties consistently as requiered
 (#4663)

Co-authored-by: Timothee Guerin <tiguerin@microsoft.com>
---
 ...r-should-be-requiered-2024-9-9-17-24-19.md |  7 ++++++
 packages/openapi3/src/schema-emitter.ts       |  7 ++++++
 packages/openapi3/test/discriminator.test.ts  | 22 +++++++++++++++----
 .../@typespec/openapi3/openapi.yaml           |  1 +
 .../@typespec/openapi3/openapi.v1.yaml        |  1 +
 .../@typespec/openapi3/openapi.v2.yaml        |  1 +
 6 files changed, 35 insertions(+), 4 deletions(-)
 create mode 100644 .chronus/changes/fix-discriminator-should-be-requiered-2024-9-9-17-24-19.md

diff --git a/.chronus/changes/fix-discriminator-should-be-requiered-2024-9-9-17-24-19.md b/.chronus/changes/fix-discriminator-should-be-requiered-2024-9-9-17-24-19.md
new file mode 100644
index 0000000000..d854fd90d3
--- /dev/null
+++ b/.chronus/changes/fix-discriminator-should-be-requiered-2024-9-9-17-24-19.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Discriminator properties are marked as required regardless if they are in TypeSpec to match OpenAPI3 spec.
diff --git a/packages/openapi3/src/schema-emitter.ts b/packages/openapi3/src/schema-emitter.ts
index 71c34c3fe9..90446bf0df 100644
--- a/packages/openapi3/src/schema-emitter.ts
+++ b/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;
   }
 
diff --git a/packages/openapi3/test/discriminator.test.ts b/packages/openapi3/test/discriminator.test.ts
index 0d2bb8f4aa..88192cd26f 100644
--- a/packages/openapi3/test/discriminator.test.ts
+++ b/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 +344,18 @@ 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;
+      kind?: string;
+     }`);
+
+    deepStrictEqual(openApi.components.schemas.Animal.required, ["id", "kind"]);
+    deepStrictEqual(openApi.components.schemas.Animal.discriminator, {
+      propertyName: "kind",
+    });
+  });
 });
diff --git a/packages/samples/test/output/polymorphism/@typespec/openapi3/openapi.yaml b/packages/samples/test/output/polymorphism/@typespec/openapi3/openapi.yaml
index 6e9172b3a2..b8a536d4cc 100644
--- a/packages/samples/test/output/polymorphism/@typespec/openapi3/openapi.yaml
+++ b/packages/samples/test/output/polymorphism/@typespec/openapi3/openapi.yaml
@@ -50,6 +50,7 @@ components:
       type: object
       required:
         - name
+        - kind
       properties:
         name:
           type: string
diff --git a/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v1.yaml b/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v1.yaml
index 8befefdcfb..48b989550b 100644
--- a/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v1.yaml
+++ b/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v1.yaml
@@ -63,6 +63,7 @@ components:
       required:
         - name
         - favoriteToys
+        - type
       properties:
         name:
           type: string
diff --git a/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v2.yaml b/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v2.yaml
index 10b519e5f0..01e3552978 100644
--- a/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v2.yaml
+++ b/packages/samples/test/output/versioning/@typespec/openapi3/openapi.v2.yaml
@@ -85,6 +85,7 @@ components:
       required:
         - name
         - favoriteToys
+        - type
       properties:
         name:
           type: string