Don't memoize association serializer

lib/active_model/serializer/associations.rb

@@ -20,11 +20,11 @@ module ActiveModel
         @embedded_key  = options[:root] || name
 
         serializer = @options[:serializer]
-        @serializer_class = serializer.is_a?(String) ? serializer.constantize : serializer
+        @serializer_from_options = serializer.is_a?(String) ? serializer.constantize : serializer
       end
 
       attr_reader :name, :embed_ids, :embed_objects
-      attr_accessor :embed_in_root, :embed_key, :key, :embedded_key, :root_key, :serializer_class, :options
+      attr_accessor :embed_in_root, :embed_key, :key, :embedded_key, :root_key, :serializer_from_options, :options
       alias embed_ids? embed_ids
       alias embed_objects? embed_objects
       alias embed_in_root? embed_in_root
@@ -34,8 +34,16 @@ module ActiveModel
         @embed_objects = embed == :object || embed == :objects
       end
 
+      def serializer_from_object(object)
+        Serializer.serializer_for(object)
+      end
+
+      def default_serializer
+        DefaultSerializer
+      end
+
       def build_serializer(object, options = {})
-        @serializer_class.new(object, options.merge(@options))
+        serializer_class(object).new(object, options.merge(self.options))
       end
 
       class HasOne < Association
@@ -45,8 +53,11 @@ module ActiveModel
           @key ||= "#{name}_id"
         end
 
+        def serializer_class(object)
+          serializer_from_options || serializer_from_object(object) || default_serializer
+        end
+
         def build_serializer(object, options = {})
-          @serializer_class ||= Serializer.serializer_for(object) || DefaultSerializer
           options[:_wrap_in_array] = embed_in_root?
           super
         end
@@ -59,17 +70,29 @@ module ActiveModel
           @key ||= "#{name.to_s.singularize}_ids"
         end
 
-        def build_serializer(object, options = {})
+        def serializer_class(object)
-          if @serializer_class && !(@serializer_class <= ArraySerializer)
+          if use_array_serializer?
-            @options[:each_serializer] = @serializer_class
+            ArraySerializer
-            @serializer_class = ArraySerializer
           else
-            @serializer_class ||= ArraySerializer
+            serializer_from_options
+          end
         end
 
+        def options
+          if use_array_serializer?
+            { each_serializer: serializer_from_options }.merge! super
+          else
             super
           end
         end
+
+        private
+
+        def use_array_serializer?
+          !serializer_from_options ||
+            serializer_from_options && !(serializer_from_options <= ArraySerializer)
+        end
+      end
     end
   end
 end

test/unit/active_model/array_serializer/serialization_test.rb

@@ -78,6 +78,36 @@ module ActiveModel
       ensure
         PostSerializer._associations[:comments] = @old_association
       end
+
+      def test_embed_object_for_has_one_association_with_nil_value
+        @association = UserSerializer._associations[:profile]
+        @old_association = @association.dup
+
+        @association.embed = :objects
+
+        @user1 = User.new({ name: 'User 1', email: 'email1@server.com' })
+        @user2 = User.new({ name: 'User 2', email: 'email2@server.com' })
+
+        class << @user1
+          def profile
+            nil
+          end
+        end
+
+        class << @user2
+          def profile
+            @profile ||= Profile.new(name: 'Name 1', description: 'Desc 1')
+          end
+        end
+
+        @serializer = ArraySerializer.new([@user1, @user2]) #, root: :posts)
+        assert_equal([
+          { name: "User 1", email: "email1@server.com", profile: nil },
+          { name: "User 2", email: "email2@server.com", profile: { name: 'Name 1', description: 'Desc 1' } }
+        ], @serializer.as_json)
+      ensure
+        UserSerializer._associations[:profile] = @old_association
+      end
     end
   end
 end

test/unit/active_model/serializer/has_many_test.rb

@@ -128,7 +128,7 @@ module ActiveModel
       def test_associations_using_a_given_serializer
         @association.embed = :ids
         @association.embed_in_root = true
-        @association.serializer_class = Class.new(ActiveModel::Serializer) do
+        @association.serializer_from_options = Class.new(ActiveModel::Serializer) do
           def content
             object.read_attribute_for_serialization(:content) + '!'
           end
@@ -145,7 +145,7 @@ module ActiveModel
       def test_associations_embedding_ids_using_a_given_array_serializer
         @association.embed = :ids
         @association.embed_in_root = true
-        @association.serializer_class = Class.new(ActiveModel::ArraySerializer) do
+        @association.serializer_from_options = Class.new(ActiveModel::ArraySerializer) do
           def serializable_object
             { my_content: ['fake'] }
           end
@@ -158,7 +158,7 @@ module ActiveModel
       end
 
       def test_associations_embedding_objects_using_a_given_array_serializer
-        @association.serializer_class = Class.new(ActiveModel::ArraySerializer) do
+        @association.serializer_from_options = Class.new(ActiveModel::ArraySerializer) do
           def serializable_object
             { my_content: ['fake'] }
           end

test/unit/active_model/serializer/has_one_test.rb

@@ -119,7 +119,7 @@ module ActiveModel
       def test_associations_embedding_ids_using_a_given_serializer
         @association.embed = :ids
         @association.embed_in_root = true
-        @association.serializer_class = Class.new(ActiveModel::Serializer) do
+        @association.serializer_from_options = Class.new(ActiveModel::Serializer) do
           def name
             'fake'
           end
@@ -134,7 +134,7 @@ module ActiveModel
       end
 
       def test_associations_embedding_objects_using_a_given_serializer
-        @association.serializer_class = Class.new(ActiveModel::Serializer) do
+        @association.serializer_from_options = Class.new(ActiveModel::Serializer) do
           def name
             'fake'
           end

test/unit/active_model/serializer/scope_test.rb

@@ -31,7 +31,7 @@ module ActiveModel
       end
 
       def test_scope_passed_through
-        @association.serializer_class = Class.new(ActiveModel::Serializer) do
+        @association.serializer_from_options = Class.new(ActiveModel::Serializer) do
           def name
             scope
           end