Add oneOf to JSON Schema (#3557)

Fix #3544.

.chronus/changes/json-schema-oneof-2024-5-11-8-56-5.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/json-schema"
+---
+
+Add support for @oneOf decorator.

docs/emitters/json-schema/reference/decorators.md

@@ -258,6 +258,22 @@ Specify that the numeric type must be a multiple of some numeric value.
 | ----- | ----------------- | -------------------------------------------------- |
 | value | `valueof numeric` | The numeric type must be a multiple of this value. |
 
+### `@oneOf` {#@TypeSpec.JsonSchema.oneOf}
+
+Specify that `oneOf` should be used instead of `anyOf` for that union.
+
+```typespec
+@TypeSpec.JsonSchema.oneOf
+```
+
+#### Target
+
+`Union | ModelProperty`
+
+#### Parameters
+
+None
+
 ### `@prefixItems` {#@TypeSpec.JsonSchema.prefixItems}
 
 Specify that the target array must begin with the provided types.

docs/emitters/json-schema/reference/index.mdx

@@ -52,6 +52,7 @@ npm install --save-peer @typespec/json-schema
 - [`@minContains`](./decorators.md#@TypeSpec.JsonSchema.minContains)
 - [`@minProperties`](./decorators.md#@TypeSpec.JsonSchema.minProperties)
 - [`@multipleOf`](./decorators.md#@TypeSpec.JsonSchema.multipleOf)
+- [`@oneOf`](./decorators.md#@TypeSpec.JsonSchema.oneOf)
 - [`@prefixItems`](./decorators.md#@TypeSpec.JsonSchema.prefixItems)
 - [`@uniqueItems`](./decorators.md#@TypeSpec.JsonSchema.uniqueItems)
 

packages/json-schema/README.md

@@ -95,6 +95,7 @@ When true, emit all references as json schema files, even if the referenced type
 - [`@minContains`](#@mincontains)
 - [`@minProperties`](#@minproperties)
 - [`@multipleOf`](#@multipleof)
+- [`@oneOf`](#@oneof)
 - [`@prefixItems`](#@prefixitems)
 - [`@uniqueItems`](#@uniqueitems)
 
@@ -348,6 +349,22 @@ Specify that the numeric type must be a multiple of some numeric value.
 | ----- | ----------------- | -------------------------------------------------- |
 | value | `valueof numeric` | The numeric type must be a multiple of this value. |
 
+#### `@oneOf`
+
+Specify that `oneOf` should be used instead of `anyOf` for that union.
+
+```typespec
+@TypeSpec.JsonSchema.oneOf
+```
+
+##### Target
+
+`Union | ModelProperty`
+
+##### Parameters
+
+None
+
 #### `@prefixItems`
 
 Specify that the target array must begin with the provided types.

packages/json-schema/generated-defs/TypeSpec.JsonSchema.ts

@@ -5,6 +5,7 @@ import type {
   Numeric,
   Scalar,
   Type,
+  Union,
 } from "@typespec/compiler";
 
 /**
@@ -43,6 +44,11 @@ export type BaseUriDecorator = (
  */
 export type IdDecorator = (context: DecoratorContext, target: Type, id: string) => void;
 
+/**
+ * Specify that `oneOf` should be used instead of `anyOf` for that union.
+ */
+export type OneOfDecorator = (context: DecoratorContext, target: Union | ModelProperty) => void;
+
 /**
  * Specify that the numeric type must be a multiple of some numeric value.
  *

packages/json-schema/generated-defs/TypeSpec.JsonSchema.ts-test.ts

@@ -13,6 +13,7 @@ import {
   $minContains,
   $minProperties,
   $multipleOf,
+  $oneOf,
   $prefixItems,
   $uniqueItems,
 } from "@typespec/json-schema";
@@ -30,6 +31,7 @@ import type {
   MinContainsDecorator,
   MinPropertiesDecorator,
   MultipleOfDecorator,
+  OneOfDecorator,
   PrefixItemsDecorator,
   UniqueItemsDecorator,
 } from "./TypeSpec.JsonSchema.js";
@@ -38,6 +40,7 @@ type Decorators = {
   $jsonSchema: JsonSchemaDecorator;
   $baseUri: BaseUriDecorator;
   $id: IdDecorator;
+  $oneOf: OneOfDecorator;
   $multipleOf: MultipleOfDecorator;
   $contains: ContainsDecorator;
   $minContains: MinContainsDecorator;
@@ -57,6 +60,7 @@ const _: Decorators = {
   $jsonSchema,
   $baseUri,
   $id,
+  $oneOf,
   $multipleOf,
   $contains,
   $minContains,

packages/json-schema/lib/main.tsp

@@ -30,6 +30,11 @@ extern dec baseUri(target: Reflection.Namespace, baseUri: valueof string);
  */
 extern dec id(target: unknown, id: valueof string);
 
+/**
+ * Specify that `oneOf` should be used instead of `anyOf` for that union.
+ */
+extern dec oneOf(target: Reflection.Union | Reflection.ModelProperty);
+
 /**
  * Specify that the numeric type must be a multiple of some numeric value.
  *

packages/json-schema/src/index.ts

@@ -29,6 +29,7 @@ import {
   MinContainsDecorator,
   MinPropertiesDecorator,
   MultipleOfDecorator,
+  OneOfDecorator,
   PrefixItemsDecorator,
   UniqueItemsDecorator,
 } from "../generated-defs/TypeSpec.JsonSchema.js";
@@ -145,6 +146,15 @@ export function getId(program: Program, target: Type) {
   return program.stateMap(idKey).get(target);
 }
 
+const oneOfKey = createStateSymbol("JsonSchema.oneOf");
+export const $oneOf: OneOfDecorator = (context: DecoratorContext, target: Type) => {
+  context.program.stateMap(oneOfKey).set(target, true);
+};
+
+export function isOneOf(program: Program, target: Type) {
+  return program.stateMap(oneOfKey).has(target);
+}
+
 const containsKey = createStateSymbol("JsonSchema.contains");
 export const $contains: ContainsDecorator = (
   context: DecoratorContext,

packages/json-schema/src/json-schema-emitter.ts

@@ -71,6 +71,7 @@ import {
   getPrefixItems,
   getUniqueItems,
   isJsonSchemaDeclaration,
+  isOneOf,
 } from "./index.js";
 import { JSONSchemaEmitterOptions, reportDiagnostic } from "./lib.js";
 export class JsonSchemaEmitter extends TypeEmitter<Record<string, any>, JSONSchemaEmitterOptions> {
@@ -174,6 +175,11 @@ export class JsonSchemaEmitter extends TypeEmitter<Record<string, any>, JSONSche
       result.default = this.#getDefaultValue(property.type, property.default);
     }
 
+    if (result.anyOf && isOneOf(this.emitter.getProgram(), property)) {
+      result.oneOf = result.anyOf;
+      delete result.anyOf;
+    }
+
     this.#applyConstraints(property, result);
 
     return result;
@@ -296,17 +302,21 @@ export class JsonSchemaEmitter extends TypeEmitter<Record<string, any>, JSONSche
   }
 
   unionDeclaration(union: Union, name: string): EmitterOutput<object> {
+    const key = isOneOf(this.emitter.getProgram(), union) ? "oneOf" : "anyOf";
+
     const withConstraints = this.#initializeSchema(union, name, {
-      anyOf: this.emitter.emitUnionVariants(union),
+      [key]: this.emitter.emitUnionVariants(union),
     });
 
     this.#applyConstraints(union, withConstraints);
     return this.#createDeclaration(union, name, withConstraints);
   }
 
   unionLiteral(union: Union): EmitterOutput<object> {
+    const key = isOneOf(this.emitter.getProgram(), union) ? "oneOf" : "anyOf";
+
     return new ObjectBuilder({
-      anyOf: this.emitter.emitUnionVariants(union),
+      [key]: this.emitter.emitUnionVariants(union),
     });
   }
 

packages/json-schema/test/unions.test.ts

@@ -88,6 +88,26 @@ describe("emitting unions", () => {
     assert.strictEqual(Foo["x-foo"], true);
   });
 
+  it("handles oneOf decorator", async () => {
+    const schemas = await emitSchema(`
+      @oneOf
+      union Foo {
+        "a",
+        "b"
+      }
+
+      model Bar {
+        @oneOf
+        prop: "a" | "b"
+      }
+    `);
+
+    const Foo = schemas["Foo.json"];
+    const Bar = schemas["Bar.json"];
+
+    assert.ok(Foo.oneOf, "Foo uses oneOf");
+    assert.ok(Bar.properties.prop.oneOf, "Bar.prop uses oneOf");
+  });
   it("handles decorators on variants", async () => {
     const schemas = await emitSchema(`
       union Foo {