use model name to determine the type

lib/active_model/serializer.rb

@@ -188,7 +188,7 @@ def id
     end
 
     def type
-      object.class.to_s.demodulize.underscore.pluralize
+      object.class.model_name.plural
     end
 
     def attributes(options = {})

test/adapter/json_api/linked_test.rb

@@ -203,7 +203,7 @@ def test_include_multiple_posts_and_linked
             assert_equal expected, alt_adapter.serializable_hash[:included]
           end
 
-          def test_ignore_model_namespace_for_linked_resource_type
+          def test_underscore_model_namespace_for_linked_resource_type
             spammy_post = Post.new(id: 123)
             spammy_post.related = [Spam::UnrelatedLink.new(id: 456)]
             serializer = SpammyPostSerializer.new(spammy_post)
@@ -212,7 +212,7 @@ def test_ignore_model_namespace_for_linked_resource_type
             expected = {
               related: {
                 data: [{
-                  type: 'unrelated_links',
+                  type: 'spam_unrelated_links',
                   id: '456'
                 }]
               }

test/fixtures/poro.rb

@@ -1,6 +1,10 @@
 class Model
   FILE_DIGEST = Digest::MD5.hexdigest(File.open(__FILE__).read)
 
+  def self.model_name
+    @_model_name ||= ActiveModel::Name.new(self)
+  end
+
   def initialize(hash={})
     @attributes = hash
   end