fix: render discriminator selector when variants omit the discriminator property

src/components/Schema/ObjectSchema.tsx

@@ -49,7 +49,17 @@ export const ObjectSchema = observer(
     const expandByDefault =
       (expandSingleSchemaField && filteredFields.length === 1) || schemasExpansionLevel >= level!;
 
-    return (
+    // The discriminator dropdown is normally attached to the field row whose name
+    // matches the discriminator property. When the variant schemas don't declare that
+    // property (e.g. a `oneOf` + `discriminator` written without `allOf` inheritance, as
+    // commonly emitted by code generators), no field matches and the selector would
+    // silently disappear, leaving only the first variant visible. In that case render the
+    // dropdown standalone so the polymorphic variants stay switchable.
+    const hasDiscriminatorField =
+      !!discriminator && filteredFields.some(field => field.name === discriminator.fieldName);
+    const showStandaloneDiscriminator = !!discriminator && !hasDiscriminatorField;
+
+    const propertiesTable = (
       <PropertiesTable>
         {showTitle && <PropertiesTableCaption>{title}</PropertiesTableCaption>}
         <tbody>
@@ -83,5 +93,19 @@ export const ObjectSchema = observer(
         </tbody>
       </PropertiesTable>
     );
+
+    if (!showStandaloneDiscriminator) {
+      return propertiesTable;
+    }
+
+    return (
+      <>
+        <DiscriminatorDropdown
+          parent={discriminator!.parentSchema}
+          enumValues={discriminator!.parentSchema.oneOf?.map(s => s.title) ?? []}
+        />
+        {propertiesTable}
+      </>
+    );
   },
 );

src/components/__tests__/DiscriminatorDropdown.test.tsx

@@ -8,9 +8,11 @@ import * as React from 'react';
 import { filterPropsDeep } from '../../utils/test-utils';
 
 import { ObjectSchema, Schema } from '../';
+import { DiscriminatorDropdown } from '../Schema/DiscriminatorDropdown';
 import { OpenAPIParser, SchemaModel } from '../../services';
 import { RedocNormalizedOptions } from '../../services/RedocNormalizedOptions';
 import * as simpleDiscriminatorFixture from './fixtures/simple-discriminator.json';
+import * as oneOfDiscriminatorNoPropFixture from './fixtures/oneof-discriminator-no-prop.json';
 
 const options = new RedocNormalizedOptions({});
 describe('Components', () => {
@@ -52,6 +54,60 @@ describe('Components', () => {
         );
         expect(filterPropsDeep(toJson(schemaView), ['field.schema.options'])).toMatchSnapshot();
       });
+
+      it('should still render the dropdown when variants do not declare the discriminator property', () => {
+        // oneOf + discriminator where the variant schemas (CardPaymentInput, CashPaymentInput)
+        // neither declare `__typename` nor inherit it via allOf. Previously the selector was
+        // attached only to a matching field row, so it silently disappeared and only the first
+        // variant was shown. The dropdown must now render standalone.
+        const parser = new OpenAPIParser(oneOfDiscriminatorNoPropFixture, undefined, options);
+        const schema = new SchemaModel(
+          parser,
+          { $ref: '#/components/schemas/PaymentInput' },
+          '#/components/schemas/PaymentInput',
+          options,
+        );
+
+        expect(schema.discriminatorProp).toEqual('__typename');
+        expect(schema.oneOf).toHaveLength(2);
+        // sanity: the active variant genuinely has no field named like the discriminator
+        expect(schema.oneOf![0].fields?.some(f => f.name === schema.discriminatorProp)).toBe(false);
+
+        const schemaView = shallow(
+          <ObjectSchema
+            schema={schema.oneOf![0]}
+            discriminator={{
+              fieldName: schema.discriminatorProp,
+              parentSchema: schema,
+            }}
+          />,
+        );
+        expect(schemaView.find(DiscriminatorDropdown)).toHaveLength(1);
+      });
+
+      it('should not render a standalone dropdown when the variant declares the discriminator property', () => {
+        // Regression guard for the standard allOf-inheritance pattern: the discriminator field
+        // exists on the variant, so the selector stays inline on that field row and no extra
+        // standalone dropdown is added at the ObjectSchema level.
+        const parser = new OpenAPIParser(simpleDiscriminatorFixture, undefined, options);
+        const schema = new SchemaModel(
+          parser,
+          { $ref: '#/components/schemas/Pet' },
+          '#/components/schemas/Pet',
+          options,
+        );
+        const schemaView = shallow(
+          <ObjectSchema
+            schema={schema.oneOf![0]}
+            discriminator={{
+              fieldName: schema.discriminatorProp,
+              parentSchema: schema,
+            }}
+          />,
+        );
+        // the inline dropdown lives inside a <Field> render prop and is not a direct child here
+        expect(schemaView.find(DiscriminatorDropdown)).toHaveLength(0);
+      });
     });
   });
 });

src/components/__tests__/fixtures/oneof-discriminator-no-prop.json

@@ -0,0 +1,39 @@
+{
+  "openapi": "3.1.0",
+  "components": {
+    "schemas": {
+      "PaymentInput": {
+        "type": "object",
+        "properties": {
+          "amount": { "type": "integer" }
+        },
+        "oneOf": [
+          { "$ref": "#/components/schemas/CardPaymentInput" },
+          { "$ref": "#/components/schemas/CashPaymentInput" }
+        ],
+        "required": ["amount"],
+        "discriminator": {
+          "propertyName": "__typename",
+          "mapping": {
+            "CardPayment": "#/components/schemas/CardPaymentInput",
+            "CashPayment": "#/components/schemas/CashPaymentInput"
+          }
+        }
+      },
+      "CardPaymentInput": {
+        "type": "object",
+        "properties": {
+          "last4": { "type": "string" }
+        },
+        "required": ["last4"]
+      },
+      "CashPaymentInput": {
+        "type": "object",
+        "properties": {
+          "tendered": { "type": "integer" }
+        },
+        "required": ["tendered"]
+      }
+    }
+  }
+}