diff --git a/lib/ams/serializer.rb b/lib/ams/serializer.rb
index 65c788a5..a09f6c3a 100644
--- a/lib/ams/serializer.rb
+++ b/lib/ams/serializer.rb
@@ -333,9 +333,5 @@ module AMS
       def method_missing(name, *args, &block)
         object.send(name, *args, &block)
       end
-
-      def respond_to_missing?(name, include_private = false)
-        object.respond_to?(name, include_private)
-      end
   end
 end
diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index 2e95cb79..898b828d 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -52,7 +52,7 @@ end
 
 class ParentModel < PlainModel
   attributes :id, :name, :description
-  associations :child_models
+  associations :child_models, :child_model
 end
 class ChildModel < PlainModel
   attributes :id, :name
diff --git a/test/unit/serializer/attributes_test.rb b/test/unit/serializer/attributes_test.rb
index 2c0a20fb..ac0a2b69 100644
--- a/test/unit/serializer/attributes_test.rb
+++ b/test/unit/serializer/attributes_test.rb
@@ -5,6 +5,7 @@ module AMS
   class Serializer
     class AttributesTest < Test
       class ParentModelSerializer < Serializer
+        # TODO: test :id is forbidden attribute name
         attribute :name
         attribute :description, key: :summary
       end
diff --git a/test/unit/serializer/object_delegation_test.rb b/test/unit/serializer/object_delegation_test.rb
new file mode 100644
index 00000000..2c25b681
--- /dev/null
+++ b/test/unit/serializer/object_delegation_test.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+require "test_helper"
+
+module AMS
+  class Serializer
+    class ObjectDelegationTest < Test
+      class ParentModelSerializer < Serializer
+      end
+
+      def setup
+        super
+        @object = ParentModel.new(id: 1)
+        class << @object
+          def delegated?
+            true
+          end
+        end
+        @serializer_class = ParentModelSerializer
+        @serializer_instance = @serializer_class.new(@object)
+      end
+
+      def test_model_delegates_method_to_object
+        assert @serializer_instance.delegated?
+      end
+
+      def test_model_delegates_respond_to_object
+        refute @serializer_instance.respond_to?(:not_delegated?)
+      end
+    end
+  end
+end
diff --git a/test/unit/serializer/relationships_test.rb b/test/unit/serializer/relationships_test.rb
index d90e6dea..cc7bfa83 100644
--- a/test/unit/serializer/relationships_test.rb
+++ b/test/unit/serializer/relationships_test.rb
@@ -5,14 +5,18 @@ module AMS
   class Serializer
     class RelationshipsTest < Test
       class ParentModelSerializer < Serializer
+        # TODO: test to: :many without :ids option
+        # TODO: test to: :one without :id option
+        # TODO: test to: :unknown_option raises ArgumentError
         relation :child_models, type: :comments, to: :many, ids: "object.child_models.map(&:id)"
+        relation :child_model, type: :comments, to: :one, id: "object.child_model.id"
       end
 
       def setup
         super
-        @relation = ChildModel.new(id: 2, name: "comment")
         @object = ParentModel.new(
-          child_models: [@relation]
+          child_models: [ ChildModel.new(id: 2, name: "comments") ],
+          child_model: ChildModel.new(id: 1, name: "comment")
         )
         @serializer_class = ParentModelSerializer
         @serializer_instance = @serializer_class.new(@object)
@@ -22,6 +26,9 @@ module AMS
         expected_relations = {
           child_models: {
             data: [{ type: "comments", id: 2 }]
+          },
+          child_model: {
+            data: { type: "comments", id: 1 }
           }
         }
         assert_equal expected_relations, @serializer_instance.relations
@@ -33,6 +40,30 @@ module AMS
         }
         assert_equal expected, @serializer_instance.relationship_data(5, :bananas)
       end
+
+      def test_model_instance_relationship_to_one
+        expected = {
+          data: { id: @object.child_model.id, type: "comments" }
+        }
+        assert_equal expected, @serializer_instance.child_model
+      end
+
+      def test_model_instance_relationship_to_one_id
+        expected = @object.child_model.id
+        assert_equal expected, @serializer_instance.related_child_model_id
+      end
+
+      def test_model_instance_relationship_to_many
+        expected = {
+          data: [{ id: @object.child_models.first.id, type: "comments" }]
+        }
+        assert_equal expected, @serializer_instance.child_models
+      end
+
+      def test_model_instance_relationship_to_many_ids
+        expected = @object.child_models.map(&:id)
+        assert_equal expected, @serializer_instance.related_child_models_ids
+      end
     end
   end
 end
diff --git a/test/unit/serializer/type_test.rb b/test/unit/serializer/type_test.rb
new file mode 100644
index 00000000..bcdb0b4c
--- /dev/null
+++ b/test/unit/serializer/type_test.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+require "test_helper"
+
+module AMS
+  class Serializer
+    class TypeTest < Test
+      class ParentModelSerializer < Serializer
+        type :something
+      end
+
+      def setup
+        super
+        @object = ParentModel.new(id: 1,)
+        @serializer_class = ParentModelSerializer
+        @serializer_instance = @serializer_class.new(@object)
+      end
+
+      def test_model_instance_type
+        expected_type = :something
+        assert_equal expected_type, @serializer_instance.type
+      end
+    end
+  end
+end