Don't treat has_one associations embed objects as an array

Closes #442

Thanks @arrtchiu for reporing and giving a test case

lib/active_model/serializer/associations.rb

@@ -19,23 +19,16 @@ module ActiveModel
         @key           = options[:key]
         @embedded_key  = options[:root] || name
 
-        self.serializer_class = @options[:serializer]
+        serializer = @options[:serializer]
+        @serializer_class = serializer.is_a?(String) ? serializer.constantize : serializer
       end
 
-      attr_reader :name, :embed_ids, :embed_objects, :serializer_class
-      attr_accessor :embed_in_root, :embed_key, :key, :embedded_key, :root_key, :options
+      attr_reader :name, :embed_ids, :embed_objects
+      attr_accessor :embed_in_root, :embed_key, :key, :embedded_key, :root_key, :serializer_class, :options
       alias embed_ids? embed_ids
       alias embed_objects? embed_objects
       alias embed_in_root? embed_in_root
 
-      def serializer_class=(serializer)
-        @serializer_class = serializer.is_a?(String) ? serializer.constantize : serializer
-        if @serializer_class && !(@serializer_class <= ArraySerializer)
-          @options.merge!(each_serializer: @serializer_class)
-          @serializer_class = ArraySerializer
-        end
-      end
-
       def embed=(embed)
         @embed_ids     = embed == :id || embed == :ids
         @embed_objects = embed == :object || embed == :objects
@@ -49,6 +42,10 @@ module ActiveModel
         end
 
         def build_serializer(object)
+          if object.respond_to?(:to_ary)
+            @options.merge!(each_serializer: @serializer_class)
+            @serializer_class = ArraySerializer
+          end
           @serializer_class ||= Serializer.serializer_for(object) || DefaultSerializer
           @serializer_class.new(object, @options)
         end
@@ -62,6 +59,10 @@ module ActiveModel
         end
 
         def build_serializer(object)
+          if @serializer_class && !(@serializer_class <= ArraySerializer)
+            @options.merge!(each_serializer: @serializer_class)
+            @serializer_class = ArraySerializer
+          end
           @serializer_class ||= ArraySerializer
           @serializer_class.new(object, @options)
         end

test/unit/active_model/serializer/has_many_test.rb

@@ -130,7 +130,7 @@ module ActiveModel
         }, @post_serializer.as_json)
       end
 
-      def test_associations_using_a_given_array_serializer
+      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
@@ -144,6 +144,18 @@ module ActiveModel
           comments: { my_content: ['fake'] }
         }, @post_serializer.as_json)
       end
+
+      def test_associations_embedding_objects_using_a_given_array_serializer
+        @association.serializer_class = Class.new(ActiveModel::ArraySerializer) do
+          def serializable_object
+            { my_content: ['fake'] }
+          end
+        end
+
+        assert_equal({
+          'post' => { title: 'Title 1', body: 'Body 1', comments: { my_content: ['fake'] } }
+        }, @post_serializer.as_json)
+      end
     end
   end
 end

test/unit/active_model/serializer/has_one_test.rb

@@ -116,7 +116,7 @@ module ActiveModel
         }, @user_serializer.as_json)
       end
 
-      def test_associations_using_a_given_serializer
+      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
@@ -132,6 +132,20 @@ module ActiveModel
           'profiles' => [{ name: 'fake' }]
         }, @user_serializer.as_json)
       end
+
+      def test_associations_embedding_objects_using_a_given_serializer
+        @association.serializer_class = Class.new(ActiveModel::Serializer) do
+          def name
+            'fake'
+          end
+
+          attributes :name
+        end
+
+        assert_equal({
+          'user' => { name: 'Name 1', email: 'mail@server.com', profile: { name: 'fake' } }
+        }, @user_serializer.as_json)
+      end
     end
   end
 end