diff --git a/lib/active_model/serializer/association/has_many.rb b/lib/active_model/serializer/association/has_many.rb
index 52d66d26..f05e9218 100644
--- a/lib/active_model/serializer/association/has_many.rb
+++ b/lib/active_model/serializer/association/has_many.rb
@@ -4,12 +4,11 @@ module ActiveModel
       class HasMany < Association
         def initialize(name, *args)
           super
-          @root_key = @embedded_key
+          @root_key = @embedded_key.to_s
           @key ||= case CONFIG.default_key_type
             when :name then name.to_s.pluralize
             else "#{name.to_s.singularize}_ids"
           end
-
         end
 
         def serializer_class(object, _)
@@ -37,4 +36,4 @@ module ActiveModel
       end
     end
   end
-end
\ No newline at end of file
+end
diff --git a/test/integration/active_record/active_record_test.rb b/test/integration/active_record/active_record_test.rb
index e65f7bb9..14ca1982 100644
--- a/test/integration/active_record/active_record_test.rb
+++ b/test/integration/active_record/active_record_test.rb
@@ -49,9 +49,9 @@ module ActiveModel
                 'ar_tag_ids' => [1, 2],
                 'ar_section_id' => 1
               },
-              ar_comments: [{ body: 'what a dumb post', 'ar_tag_ids' => [3, 2] },
+              'ar_comments' => [{ body: 'what a dumb post', 'ar_tag_ids' => [3, 2] },
                             { body: 'i liked it', 'ar_tag_ids' => [3, 1] }],
-              ar_tags: [{ name: 'happy' }, { name: 'whiny' }, { name: 'short' }],
+              'ar_tags' => [{ name: 'happy' }, { name: 'whiny' }, { name: 'short' }],
               'ar_sections' => [{ 'name' => 'ruby' }]
             }, post_serializer.as_json)
           end
diff --git a/test/unit/active_model/array_serializer/serialization_test.rb b/test/unit/active_model/array_serializer/serialization_test.rb
index 01f55b30..4d3a71dc 100644
--- a/test/unit/active_model/array_serializer/serialization_test.rb
+++ b/test/unit/active_model/array_serializer/serialization_test.rb
@@ -85,7 +85,7 @@ module ActiveModel
               {title: "Title 1", body: "Body 1", "comment_ids" => @post1.comments.map(&:object_id) },
               {title: "Title 2", body: "Body 2", "comment_ids" => @post2.comments.map(&:object_id) }
             ],
-            comments: [
+            'comments' => [
               {content: "C1"},
               {content: "C2"},
               {content: "C3"},
diff --git a/test/unit/active_model/serializer/has_many_polymorphic_test.rb b/test/unit/active_model/serializer/has_many_polymorphic_test.rb
index e5180cfe..d25a0988 100644
--- a/test/unit/active_model/serializer/has_many_polymorphic_test.rb
+++ b/test/unit/active_model/serializer/has_many_polymorphic_test.rb
@@ -140,7 +140,7 @@ module ActiveModel
               { id: c.object_id, type: model_name(c) }
             end,
           },
-          attachments: [
+          'attachments' => [
             { type: :image, image: { url: 'U1' }},
             { type: :video, video: { html: 'H1' }}
           ]
@@ -153,7 +153,7 @@ module ActiveModel
 
         assert_equal({
           'mail' => { body: 'Body 1' },
-          attachments: [
+          'attachments' => [
             { type: :image, image: { url: 'U1' }},
             { type: :video, video: { html: 'H1' }}
           ]
@@ -178,7 +178,7 @@ module ActiveModel
               { id: c.object_id, type: model_name(c) }
             end
           },
-          attachments: [
+          'attachments' => [
             { type: :image, image: { fake: 'fake' }},
             { type: :video, video: { fake: 'fake' }}
           ]
diff --git a/test/unit/active_model/serializer/has_many_test.rb b/test/unit/active_model/serializer/has_many_test.rb
index f8b2c738..c6699150 100644
--- a/test/unit/active_model/serializer/has_many_test.rb
+++ b/test/unit/active_model/serializer/has_many_test.rb
@@ -111,7 +111,7 @@ module ActiveModel
 
         assert_equal({
           'post' => { title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } },
-          comments: [{ content: 'C1' }, { content: 'C2' }]
+          'comments' => [{ content: 'C1' }, { content: 'C2' }]
         }, @post_serializer.as_json)
       end
 
@@ -128,7 +128,7 @@ module ActiveModel
             name: 'Name 1',
             posts: [{ title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } }]
           },
-          comments: [{ content: 'C1' }, { content: 'C2' }]
+          "comments" => [{ content: 'C1' }, { content: 'C2' }]
         }, category_serializer.as_json)
       end
 
@@ -138,7 +138,7 @@ module ActiveModel
 
         assert_equal({
           'post' => { title: 'Title 1', body: 'Body 1' },
-          comments: [{ content: 'C1' }, { content: 'C2' }]
+          'comments' => [{ content: 'C1' }, { content: 'C2' }]
         }, @post_serializer.as_json)
       end
 
@@ -155,7 +155,7 @@ module ActiveModel
 
         assert_equal({
           'post' => { title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } },
-          comments: [{ content: 'C1!' }, { content: 'C2!' }]
+          'comments' => [{ content: 'C1!' }, { content: 'C2!' }]
         }, @post_serializer.as_json)
       end
 
@@ -170,7 +170,7 @@ module ActiveModel
 
         assert_equal({
           'post' => { title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } },
-          comments: { my_content: ['fake'] }
+          'comments' => { my_content: ['fake'] }
         }, @post_serializer.as_json)
       end
 
@@ -191,16 +191,16 @@ module ActiveModel
         @association.embed_in_root_key = :linked
         @association.embed = :ids
         assert_equal({
+          'post' => {
+            title: 'Title 1', body: 'Body 1',
+            'comment_ids' => @post.comments.map(&:object_id)
+          },
           linked: {
-            comments: [
+            'comments' => [
               { content: 'C1' },
               { content: 'C2' }
             ]
           },
-          'post' => {
-            title: 'Title 1', body: 'Body 1',
-            'comment_ids' => @post.comments.map(&:object_id)
-          }
         }, @post_serializer.as_json)
       end
 
@@ -211,18 +211,18 @@ module ActiveModel
         @association.embed_namespace = :links
         @association.key = :comments
         assert_equal({
-          linked: {
-            comments: [
-              { content: 'C1' },
-              { content: 'C2' }
-            ]
-          },
           'post' => {
             title: 'Title 1', body: 'Body 1',
             links: {
               comments: @post.comments.map(&:object_id)
             }
-          }
+          },
+          linked: {
+            'comments' => [
+              { content: 'C1' },
+              { content: 'C2' }
+            ]
+          },
         }, @post_serializer.as_json)
       end