test: failing request posting sti with polymorphic has one

test/fixtures/active_record.rb

@@ -275,6 +275,7 @@
     t.string :drive_layout
     t.string :serial_number
     t.integer :person_id
+    t.references :imageable, polymorphic: true, index: true
     t.timestamps null: false
   end
 
@@ -734,6 +735,9 @@ class Picture < ActiveRecord::Base
 
 class Vehicle < ActiveRecord::Base
   belongs_to :person
+  belongs_to :imageable, polymorphic: true
+  # belongs_to :document, -> { where( pictures: { imageable_type: 'Document' } ) }, foreign_key: 'imageable_id'
+  # belongs_to :product, -> { where( pictures: { imageable_type: 'Product' } ) }, foreign_key: 'imageable_id'
 end
 
 class Car < Vehicle
@@ -743,13 +747,13 @@ class Boat < Vehicle
 end
 
 class Document < ActiveRecord::Base
-  has_many :pictures, as: :imageable
+  has_many :pictures, as: :imageable # polymorphic
   belongs_to :author, class_name: 'Person', foreign_key: 'author_id'
   has_one :file_properties, as: :fileable
 end
 
 class Product < ActiveRecord::Base
-  has_many :pictures, as: :imageable
+  has_many :pictures, as: :imageable # polymorphic
   belongs_to :designer, class_name: 'Person', foreign_key: 'designer_id'
   has_one :file_properties, as: :fileable
 end
@@ -1336,6 +1340,7 @@ class VehicleResource < JSONAPI::Resource
   immutable
 
   has_one :person
+  has_one :imageable, polymorphic: true
   attributes :make, :model, :serial_number
 end
 
@@ -1915,6 +1920,8 @@ class PreferencesResource < PreferencesResource; end
     class SectionResource < SectionResource; end
     class TagResource < TagResource; end
     class CommentResource < CommentResource; end
+    class DocumentResource < DocumentResource; end
+    class ProductResource < ProductResource; end
     class VehicleResource < VehicleResource; end
     class CarResource < CarResource; end
     class BoatResource < BoatResource; end

test/integration/requests/request_test.rb

@@ -359,6 +359,47 @@ def test_post_polymorphic_with_has_many_relationship
     assert_equal Car, person.vehicles.fourth.class
   end
 
+  def test_post_sti_polymorphic_with_has_one_relationship
+    post '/vehicles', params:
+      {
+        'data' => {
+          'type' => 'cars',
+          'attributes' => {
+            'make' => 'Mazda',
+            'model' => 'Miata MX5',
+            'drive_layout' => 'Front Engine RWD',
+            'serial_number' => '32432adfsfdysua',
+          },
+          'relationships' => {
+            'person' => {
+              'data' => {
+                'type' => 'people', 'id' => '1001',
+              }
+            },
+            'picture' => {
+              'data' => {
+                'type' => 'products', 'id' => '1',
+              }
+            },
+          }
+        }
+      }.to_json,
+      headers: {
+        'CONTENT_TYPE' => JSONAPI::MEDIA_TYPE,
+        'Accept' => JSONAPI::MEDIA_TYPE
+      }
+
+    assert_jsonapi_response 201
+
+    body = JSON.parse(response.body)
+    car = Vehicle.find(body.dig("data", "id"))
+
+    assert_equal "Card", car.type
+    assert_equal "Mazda", car.make
+    assert_equal Product, car.picture.class
+    assert_equal Person, car.person.class
+  end
+
   def test_post_polymorphic_invalid_with_wrong_type
     post '/people', params:
       {