* Configure adapter using ActiveModel::Serializer.config.adapter

* Get adapter instance using ActiveModel::Serializer::Adapter.adapter_for(serializer)

test/adapter_test.rb

@@ -21,5 +21,54 @@ module ActiveModel
         end
       end
     end
+
+    class AdapterForTest < Minitest::Test
+      def setup
+        profile = Profile.new
+        @serializer = ProfileSerializer.new(profile)
+        @previous_adapter = ActiveModel::Serializer.config.adapter
+      end
+
+      def teardown
+        ActiveModel::Serializer.config.adapter = @previous_adapter
+      end
+
+      def test_returns_default_adapter
+        adapter = Adapter.adapter_for(@serializer)
+        assert_kind_of ActiveModel::Serializer::Adapter::SimpleAdapter, adapter
+      end
+
+      def test_overwrite_adapter_with_symbol
+        ActiveModel::Serializer.config.adapter = :null
+
+        adapter = Adapter.adapter_for(@serializer)
+        assert_kind_of ActiveModel::Serializer::Adapter::NullAdapter, adapter
+      ensure
+
+      end
+
+      def test_overwrite_adapter_with_class
+        ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::NullAdapter
+
+        adapter = Adapter.adapter_for(@serializer)
+        assert_kind_of ActiveModel::Serializer::Adapter::NullAdapter, adapter
+      end
+
+      def test_raises_exception_if_invalid_symbol_given
+        ActiveModel::Serializer.config.adapter = :unknown
+
+        assert_raises ArgumentError do
+          Adapter.adapter_for(@serializer)
+        end
+      end
+
+      def test_raises_exception_if_it_does_not_know_hot_to_infer_adapter
+        ActiveModel::Serializer.config.adapter = 42
+
+        assert_raises ArgumentError do
+          Adapter.adapter_for(@serializer)
+        end
+      end
+    end
   end
 end