Fix polymorphic type setter

lib/jsonapi/resource.rb

@@ -324,7 +324,7 @@ def _replace_polymorphic_to_one_link(relationship_type, key_value, key_type, opt
       relationship = self.class._relationships[relationship_type.to_sym]
 
       _model.public_send("#{relationship.foreign_key}=", key_value)
-      _model.public_send("#{relationship.polymorphic_type}=", key_type.to_s.classify)
+      _model.public_send("#{relationship.polymorphic_type}=", _model_class_name(key_type))
 
       @save_needed = true
 
@@ -404,6 +404,12 @@ def _replace_fields(field_data)
       :completed
     end
 
+    def _model_class_name(key_type)
+      type_class_name = key_type.to_s.classify
+      resource = self.class.resource_for(type_class_name)
+      resource ? resource._model_name.to_s : type_class_name
+    end
+
     class << self
       def inherited(subclass)
         subclass.abstract(false)

test/fixtures/active_record.rb

@@ -572,6 +572,9 @@ class Document < ActiveRecord::Base
   has_many :pictures, as: :imageable
 end
 
+class Document::Topic < Document
+end
+
 class Product < ActiveRecord::Base
   has_one :picture, as: :imageable
 end
@@ -1209,6 +1212,11 @@ class DocumentResource < JSONAPI::Resource
   has_many :pictures
 end
 
+class TopicResource < JSONAPI::Resource
+  model_name 'Document::Topic'
+  has_many :pictures
+end
+
 class ProductResource < JSONAPI::Resource
   attribute :name
   has_one :picture, always_include_linkage_data: true

test/unit/resource/resource_test.rb

@@ -205,6 +205,14 @@ def test_class_relationships
     assert_equal(relationships.size, 2)
   end
 
+  def test_replace_polymorphic_to_one_link
+    picture_resource = PictureResource.find_by_key(Picture.first)
+    picture_resource.replace_polymorphic_to_one_link('imageable', '9', 'Topic')
+
+    assert Picture.first.imageable_id == 9
+    assert Picture.first.imageable_type == Document::Topic.to_s
+  end
+
   def test_duplicate_relationship_name
     assert_output nil, "[DUPLICATE RELATIONSHIP] `mother` has already been defined in CatResource.\n" do
       CatResource.instance_eval do