Serialize associations that doesn't have an associated serializer

lib/active_model/array_serializer.rb

@@ -1,3 +1,4 @@
+require 'active_model/default_serializer'
 require 'active_model/serializable'
 require 'active_model/serializer'
 
@@ -22,12 +23,8 @@ module ActiveModel
 
     def serializable_array
       @object.map do |item|
-        serializer = @options[:each_serializer] || Serializer.serializer_for(item)
-        if serializer
-          serializer.new(item).serializable_object(@options.merge(root: nil))
-        else
-          item.as_json
-        end
+        serializer = @options[:each_serializer] || Serializer.serializer_for(item) || DefaultSerializer
+        serializer.new(item).serializable_object(@options.merge(root: nil))
       end
     end
     alias serializable_object serializable_array

lib/active_model/default_serializer.rb

@@ -0,0 +1,17 @@
+module ActiveModel
+  # DefaultSerializer
+  #
+  # Provides a constant interface for all items
+  class DefaultSerializer
+    attr_reader :object
+
+    def initialize(object, options=nil)
+      @object = object
+    end
+
+    def serializable_hash(*)
+      @object.as_json
+    end
+    alias serializable_object serializable_hash
+  end
+end

lib/active_model/serializer/associations.rb

@@ -1,3 +1,4 @@
+require 'active_model/default_serializer'
 require 'active_model/serializer'
 
 module ActiveModel
@@ -32,13 +33,8 @@ module ActiveModel
       end
 
       def build_serializer(object)
-        @serializer_class ||= Serializer.serializer_for(object)
-
-        if @serializer_class
-          @serializer_class.new(object, @options)
-        else
-          object
-        end
+        @serializer_class ||= Serializer.serializer_for(object) || DefaultSerializer
+        @serializer_class.new(object, @options)
       end
 
       class HasOne < Association

test/unit/active_model/default_serializer_test.rb

@@ -0,0 +1,15 @@
+require 'test_helper'
+
+module ActiveModel
+  class DefaultSerializer
+    class Test < ActiveModel::TestCase
+      def test_serialize_objects
+        assert_equal(nil, DefaultSerializer.new(nil).serializable_hash)
+        assert_equal(1, DefaultSerializer.new(1).serializable_hash)
+        assert_equal('hi', DefaultSerializer.new('hi').serializable_hash)
+        obj = Struct.new(:a, :b).new(1, 2)
+        assert_equal({ a: 1, b: 2 }, DefaultSerializer.new(obj).serializable_hash)
+      end
+    end
+  end
+end

test/unit/active_model/serializer/has_many_test.rb

@@ -54,6 +54,19 @@ module ActiveModel
         }, @post_serializer.as_json)
       end
 
+      def test_associations_embedding_nil_objects_serialization_using_as_json
+        @association.embed = :objects
+        @post.instance_eval do
+          def comments
+            [nil]
+          end
+        end
+
+        assert_equal({
+          'title' => 'Title 1', 'body' => 'Body 1', 'comments' => [nil]
+        }, @post_serializer.as_json)
+      end
+
       def test_associations_embedding_objects_serialization_using_serializable_hash_and_root_from_options
         @association.embed = :objects
         @association.embedded_key = 'root'

test/unit/active_model/serializer/has_one_test.rb

@@ -54,6 +54,19 @@ module ActiveModel
         }, @user_serializer.as_json)
       end
 
+      def test_associations_embedding_nil_objects_serialization_using_as_json
+        @association.embed = :objects
+        @user.instance_eval do
+          def profile
+            nil
+          end
+        end
+
+        assert_equal({
+          'name' => 'Name 1', 'email' => 'mail@server.com', 'profiles' => [nil]
+        }, @user_serializer.as_json)
+      end
+
       def test_associations_embedding_objects_serialization_using_serializable_hash_and_root_from_options
         @association.embed = :objects
         @association.embedded_key = 'root'