Tests for ActiveModel::Serializer.serializer_for for not array serializer

test/serializers/serializer_for_test.rb

@@ -3,6 +3,7 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class SerializerForTest < Minitest::Test
+      class ArraySerializerTest < Minitest::Test
         def setup
           @array = [1, 2, 3]
           @previous_array_serializer = ActiveModel::Serializer.config.array_serializer
@@ -24,5 +25,23 @@ module ActiveModel
           assert_equal new_array_serializer, serializer
         end
       end
+
+      class SerializerTest <  Minitest::Test
+        def setup
+          @profile = Profile.new
+          @model = ::Model.new
+        end
+
+        def test_serializer_for_existing_serializer
+          serializer = ActiveModel::Serializer.serializer_for(@profile)
+          assert_equal ProfileSerializer, serializer
+        end
+
+        def test_serializer_for_not_existing_serializer
+          serializer = ActiveModel::Serializer.serializer_for(@model)
+          assert_equal nil, serializer
+        end
+      end
+    end
   end
 end