remove incorrect restriction in discriminator

This restriction was also added and then removed from the dialect metaschema,
due to a misunderstanding of the spec.

see OAI/OpenAPI-Specification#3405

Also handle the ambiguity between named schema and uri-reference in mapping
entries.

Author: karenetheridge

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

@@ -48,9 +48,6 @@ sub _traverse_keyword_discriminator ($self, $schema, $state) {
     }
   }
 
-  $valid = E({ %$state, keyword => undef }, 'missing sibling keyword to discriminator: one of oneOf, anyOf, allOf')
-    if not grep exists $schema->{$_}, qw(oneOf anyOf allOf);
-
   return 1;
 }
 
@@ -67,15 +64,21 @@ sub _eval_keyword_discriminator ($self, $data, $schema, $state) {
   my $discriminator_value = $data->{$discriminator_key};
 
   # if /components/$discriminator_value exists, that schema must validate
-  my $uri = Mojo::URL->new->fragment(jsonp('', qw(components schemas), $discriminator_value))
+  my $uri = Mojo::URL->new->fragment(jsonp('/components/schemas', $discriminator_value))
     ->to_abs($state->{initial_schema_uri});
   if (my $component_schema_info = $state->{evaluator}->_fetch_from_uri($uri)) {
     $state = { %$state, _schema_path_suffix => 'propertyName' };
   }
   elsif (exists $schema->{discriminator}{mapping} and exists $schema->{discriminator}{mapping}{$discriminator_value}) {
     # use 'mapping' to determine which schema to use.
-    $uri = Mojo::URL->new($schema->{discriminator}{mapping}{$discriminator_value})
+    # Note that the spec uses an example that assumes that the mapping value can be appended to
+    # /components/schemas/, _as well as_ being treated as a uri-reference, but this is ambiguous.
+    # For now we will handle it by preprending '#/components/schemas' if it is not already a
+    # fragment-only uri reference.
+    my $mapping = $schema->{discriminator}{mapping}{$discriminator_value};
+    $uri = Mojo::URL->new($mapping =~ /^#/ ? $mapping : '#/components/schemas/'.$mapping)
       ->to_abs($state->{initial_schema_uri});
+
     $state = { %$state, _schema_path_suffix => [ 'mapping', $discriminator_value ] };
   }
   else {

t/discriminator.t

@@ -178,19 +178,18 @@ YAML
 };
 
 subtest 'discriminator in a parent definition' => sub {
-# 8< start of temporary exception-catch
-  local $TODO = 'requirement of sibling oneOf/anyOf/allOf is not actually in the spec';
-
-  my $openapi;
-  eval {
-# >8 end of temporary exception-catch
-  $openapi = OpenAPI::Modern->new(
+  my $openapi = OpenAPI::Modern->new(
     openapi_uri => '/api',
     openapi_schema => $yamlpp->load_string(<<YAML));
 $openapi_preamble
   description: 'runtime: use discriminator to determine petType'
 components:
   schemas:
+    petType:
+      discriminator:
+        propertyName: petType
+        mapping:
+          dog: Dog
     Pet:
       type: object
       required:
@@ -199,10 +198,6 @@ components:
       properties:
         petType:
           type: string
-      discriminator:
-        propertyName: petType
-        mapping:
-          dog: Dog
     Cat:
       allOf:
       - \$ref: '#/components/schemas/Pet'
@@ -233,28 +228,14 @@ components:
             const: 'null'
 YAML
 
-  # currently we get:
-  # instance_location => '',
-  # keyword_location => '/components/schemas/Pet',
-  # absolute_keyword_location => '/api#/components/schemas/Pet',
-  # error => 'missing sibling keyword to discriminator: one of oneOf, anyOf, allOf',
   cmp_deeply(
     $openapi->evaluator->evaluate(
-      { petType => 'Cat', sound => 'meow' },
-      Mojo::URL->new('/api#/components/schemas/Pet'),
+      { petType => 'dog', sound => 'bark' },
+      Mojo::URL->new('/api#/components/schemas/petType'),
     )->TO_JSON,
     { valid => true },
     'discriminator can be defined in the base class',
   );
-
-# 8< start of temporary exception-catch
-  };
-  is(
-    $@,
-    '',
-    'no exception for this use of discriminator',
-  );
-# >8 end of temporary exception-catch
 };
 
 done_testing;