fall back to "defaultMapping" in discriminator

...when the propertyName is not present in the data payload

This keyword is new in 3.2.0, allowing the property named by the propertyName
keyword (still required!) to be absent in the payload

see also OAI/OpenAPI-Specification#5227

Author: karenetheridge

Changes

@@ -1,6 +1,7 @@
 Revision history for OpenAPI-Modern
 
 {{$NEXT}}
+          - support "defaultMapping" in the 3.2.0 discriminator object
 
 0.128     2026-02-21 23:11:29Z
           - header parameters now support Unicode header names and

lib/JSON/Schema/Modern/Vocabulary/OpenAPI.pm

@@ -59,13 +59,18 @@ sub _eval_keyword_discriminator ($self, $data, $schema, $state) {
   # Note: the spec is unclear of the expected behaviour when the data instance is not an object
   return 1 if not is_type('object', $data);
 
+  # v3.1.2 §4.8.25.1: "This property SHOULD be required in the payload schema, as the behavior when
+  # the property is absent is undefined."
+  # v3.2.0 §4.25.1: "The discriminating property MAY be defined as required or optional, but when
+  # defined as optional the Discriminator Object MUST include a defaultMapping field that specifies
+  # which schema is expected to validate the structure of the model when the discriminating property
+  # is not present."
   my $discriminator_key = $schema->{discriminator}{propertyName};
+  return E($state, 'missing required discriminator property "%s" and no defaultMapping present',
+      $discriminator_key)
+    if not exists $data->{$discriminator_key} and not exists $schema->{discriminator}{defaultMapping};
 
-  # property with name <propertyName> MUST be present in the data payload
-  return E($state, 'missing required discriminator property "%s"', $discriminator_key)
-    if not exists $data->{$discriminator_key};
-
-  my $discriminator_value = $data->{$discriminator_key};
+  my $discriminator_value = $data->{$discriminator_key} // $schema->{discriminator}{defaultMapping};
 
   if (exists $schema->{discriminator}{mapping} and exists $schema->{discriminator}{mapping}{$discriminator_value}) {
     # use 'mapping' to determine which schema to use.

t/discriminator.t

@@ -63,7 +63,7 @@ YAML
           instanceLocation => '',
           keywordLocation => '/discriminator',
           absoluteKeywordLocation => $doc_uri.'#/components/schemas/pet/discriminator',
-          error => 'missing required discriminator property "petType"',
+          error => 'missing required discriminator property "petType" and no defaultMapping present',
         },
       ],
     },
@@ -171,6 +171,32 @@ YAML
     },
     'no mapping for petType found',
   );
+
+
+  my $schema = $openapi->openapi_schema;
+  $schema->{components}{schemas}{pet}{discriminator}{defaultMapping} = 'generic_pet';
+  $schema->{components}{schemas}{pet}{anyOf}[2] = { '$ref' => '#/components/schemas/generic_pet' };
+  $schema->{components}{schemas}{generic_pet} = {
+    properties => {
+      swims => { const => false },
+      meow => { const => false },
+    },
+  };
+
+  $openapi = OpenAPI::Modern->new(openapi_uri => $doc_uri, openapi_schema => $schema);
+
+  cmp_result(
+    $openapi->evaluator->evaluate(
+      {
+        # petType not provided - use defaultMapping
+        meow => false,
+        swims => false,
+      },
+      $doc_uri->clone->fragment('/components/schemas/pet'),
+    )->TO_JSON,
+    { valid => true },
+    'defaultMapping is used as a fallback',
+  );
 };
 
 subtest 'discriminator in a parent definition' => sub {