diff --git a/lib/jsonapi/acts_as_resource_controller.rb b/lib/jsonapi/acts_as_resource_controller.rb
index 1ef1707c..e87a31d6 100644
--- a/lib/jsonapi/acts_as_resource_controller.rb
+++ b/lib/jsonapi/acts_as_resource_controller.rb
@@ -164,7 +164,7 @@ def base_url
     end
 
     def resource_klass_name
-      @resource_klass_name ||= "#{self.class.name.underscore.sub(/_controller$/, '').singularize}_resource".camelize
+      @resource_klass_name ||= "#{self.class.name.underscore.sub(/_controller$/, '').classify}Resource"
     end
 
     def verify_content_type_header
diff --git a/lib/jsonapi/link_builder.rb b/lib/jsonapi/link_builder.rb
index 23b94a4d..63b160fc 100644
--- a/lib/jsonapi/link_builder.rb
+++ b/lib/jsonapi/link_builder.rb
@@ -92,7 +92,7 @@ def build_engine
 
       begin
         unless scopes.empty?
-          "#{ scopes.first.to_s.camelize }::Engine".safe_constantize
+          "#{ scopes.first.to_s.classify }::Engine".safe_constantize
         end
 
           # :nocov:
diff --git a/lib/jsonapi/relationship.rb b/lib/jsonapi/relationship.rb
index 9d7ffc46..bff416da 100644
--- a/lib/jsonapi/relationship.rb
+++ b/lib/jsonapi/relationship.rb
@@ -155,7 +155,7 @@ class ToOne < Relationship
 
       def initialize(name, options = {})
         super
-        @class_name = options.fetch(:class_name, name.to_s.camelize)
+        @class_name = options.fetch(:class_name, name.to_s.classify)
         @foreign_key ||= "#{name}_id".to_sym
         @foreign_key_on = options.fetch(:foreign_key_on, :self)
         # if parent_resource
@@ -231,7 +231,7 @@ class ToMany < Relationship
 
       def initialize(name, options = {})
         super
-        @class_name = options.fetch(:class_name, name.to_s.camelize.singularize)
+        @class_name = options.fetch(:class_name, name.to_s.classify)
         @foreign_key ||= "#{name.to_s.singularize}_ids".to_sym
         @reflect = options.fetch(:reflect, true) == true
         # if parent_resource
diff --git a/lib/jsonapi/resource_common.rb b/lib/jsonapi/resource_common.rb
index b841b732..9ec4e0d1 100644
--- a/lib/jsonapi/resource_common.rb
+++ b/lib/jsonapi/resource_common.rb
@@ -561,7 +561,7 @@ def resource_klass_for_model(model)
       end
 
       def _resource_name_from_type(type)
-        "#{type.to_s.underscore.singularize}_resource".camelize
+        "#{type.to_s.classify}Resource"
       end
 
       def resource_type_for(model)
@@ -578,6 +578,10 @@ def resource_type_for(model)
         end
       end
 
+      def polymorphic_type_for(model_name)
+        model_name&.to_s&.classify
+      end
+
       attr_accessor :_attributes,
                     :_relationships,
                     :_type,
@@ -1200,12 +1204,12 @@ def define_relationship_methods(relationship_name, relationship_klass, options)
       def define_foreign_key_setter(relationship)
         if relationship.polymorphic?
           define_on_resource "#{relationship.foreign_key}=" do |v|
-            _model.method("#{relationship.foreign_key}=").call(v[:id])
-            _model.public_send("#{relationship.polymorphic_type}=", v[:type])
+            _model.public_send("#{relationship.foreign_key}=", v[:id])
+            _model.public_send("#{relationship.polymorphic_type}=", self.class.polymorphic_type_for(v[:type]))
           end
         else
           define_on_resource "#{relationship.foreign_key}=" do |value|
-            _model.method("#{relationship.foreign_key}=").call(value)
+            _model.public_send("#{relationship.foreign_key}=", value)
           end
         end
         relationship.foreign_key
diff --git a/test/fixtures/active_record.rb b/test/fixtures/active_record.rb
index c08aa896..92281e2a 100644
--- a/test/fixtures/active_record.rb
+++ b/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
diff --git a/test/integration/requests/request_test.rb b/test/integration/requests/request_test.rb
index aa9989c6..d6bdf7f0 100644
--- a/test/integration/requests/request_test.rb
+++ b/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 '/cars', 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',
+              }
+            },
+            'imageable' => {
+              'data' => {
+                'type' => 'products', 'id' => '1',
+              }
+            },
+          }
+        }
+      }.to_json,
+      headers: {
+        'CONTENT_TYPE' => JSONAPI::MEDIA_TYPE,
+        'Accept' => JSONAPI::MEDIA_TYPE
+      }
+
+    assert_jsonapi_response 201
+
+    body = response.parsed_body
+    car = Vehicle.find(body.dig("data", "id"))
+
+    assert_equal "Car", car.type
+    assert_equal "Mazda", car.make
+    assert_equal Product, car.imageable.class
+    assert_equal Person, car.person.class
+  end
+
   def test_post_polymorphic_invalid_with_wrong_type
     post '/people', params:
       {