embed_in_root works even if serializer is invoked through another serializers' association which itself does not have embed_in_root set

lib/active_model/serializer.rb

@@ -194,12 +194,15 @@ end
       included_associations = filter(associations.keys)
       associations.each_with_object({}) do |(name, association), hash|
         if included_associations.include? name
+          association_serializer = build_serializer(association)
+          # we must do this always because even if the current association is not
+          # embeded in root, it might have its own associations that are embeded in root
+          hash.merge!(association_serializer.embedded_in_root_associations) {|key, oldval, newval| [newval, oldval].flatten }
+
           if association.embed_in_root?
             if association.embed_in_root_key?
               hash = hash[association.embed_in_root_key] ||= {}
             end
-            association_serializer = build_serializer(association)
-            hash.merge!(association_serializer.embedded_in_root_associations) {|key, oldval, newval| [newval, oldval].flatten }
 
             serialized_data = association_serializer.serializable_object
             key = association.root_key

test/fixtures/poro.rb

@@ -22,9 +22,22 @@ class User < Model
   end
 end
 
+class UserInfo < Model
+  def user
+    @user ||= User.new(name: 'N1', email: 'E1')
+  end
+end
+
 class Profile < Model
 end
 
+class Category < Model
+  def posts
+    @posts ||= [Post.new(title: 'T1', body: 'B1'),
+                Post.new(title: 'T2', body: 'B2')]
+  end
+end
+
 class Post < Model
   def comments
     @comments ||= [Comment.new(content: 'C1'),
@@ -47,6 +60,10 @@ class UserSerializer < ActiveModel::Serializer
   has_one :profile
 end
 
+class UserInfoSerializer < ActiveModel::Serializer
+  has_one :user
+end
+
 class ProfileSerializer < ActiveModel::Serializer
   def description
     description = object.read_attribute_for_serialization(:description)
@@ -56,6 +73,12 @@ class ProfileSerializer < ActiveModel::Serializer
   attributes :name, :description
 end
 
+class CategorySerializer < ActiveModel::Serializer
+  attributes :name
+
+  has_many :posts
+end
+
 class PostSerializer < ActiveModel::Serializer
   attributes :title, :body
 

test/unit/active_model/serializer/has_many_test.rb

@@ -115,6 +115,23 @@ module ActiveModel
         }, @post_serializer.as_json)
       end
 
+      def test_associations_embedding_ids_including_objects_serialization_when_invoked_from_parent_serializer
+        @association.embed = :ids
+        @association.embed_in_root = true
+
+        category = Category.new(name: 'Name 1')
+        category.instance_variable_set(:@posts, [@post])
+        category_serializer = CategorySerializer.new(category)
+
+        assert_equal({
+          'category' => {
+            name: 'Name 1',
+            posts: [{ title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } }]
+          },
+          comments: [{ content: 'C1' }, { content: 'C2' }]
+        }, category_serializer.as_json)
+      end
+
       def test_associations_embedding_nothing_including_objects_serialization_using_as_json
         @association.embed = nil
         @association.embed_in_root = true

test/unit/active_model/serializer/has_one_test.rb

@@ -131,6 +131,20 @@ module ActiveModel
         }, @user_serializer.as_json)
       end
 
+      def test_associations_embedding_ids_including_objects_serialization_when_invoked_from_parent_serializer
+        @association.embed = :ids
+        @association.embed_in_root = true
+
+        user_info = UserInfo.new
+        user_info.instance_variable_set(:@user, @user)
+        user_info_serializer = UserInfoSerializer.new(user_info)
+
+        assert_equal({
+          'user_info' => { user: { name: 'Name 1', email: 'mail@server.com', 'profile_id' => @user.profile.object_id } },
+          'profiles' => [{ name: 'N1', description: 'D1' }]
+        }, user_info_serializer.as_json)
+      end
+
       def test_associations_embedding_ids_using_a_given_serializer
         @association.embed = :ids
         @association.embed_in_root = true