has_one serialized objects shouldn't be wrapped in an array and it's key is singular

lib/active_model/serializer.rb

@@ -152,7 +152,8 @@ module ActiveModel
       if associated_data.respond_to?(:to_ary)
         associated_data.map { |elem| association.build_serializer(elem).serializable_hash }
       else
-        [association.build_serializer(associated_data).serializable_hash]
+        result = association.build_serializer(associated_data).serializable_hash
+        association.is_a?(Association::HasMany) ? [result] : result
       end
     end
 

lib/active_model/serializer/associations.rb

@@ -12,7 +12,7 @@ module ActiveModel
         @embed_in_root = @embed_ids && (options[:include] || SETTINGS[:include])
         @embed_key     = options[:embed_key] || :id
         @key           = options[:key]
-        @embedded_key  = options[:root]
+        @embedded_key  = options[:root] || name
 
         self.serializer_class = @options[:serializer]
       end
@@ -41,7 +41,6 @@ module ActiveModel
         def initialize(*args)
           super
           @key  ||= "#{name}_id"
-          @embedded_key ||= name.pluralize
         end
       end
 
@@ -49,7 +48,6 @@ module ActiveModel
         def initialize(*args)
           super
           @key ||= "#{name.singularize}_ids"
-          @embedded_key ||= name
         end
       end
     end

test/unit/active_model/serializer/has_many_test.rb

@@ -43,14 +43,14 @@ module ActiveModel
       def test_associations_embedding_objects_serialization_using_serializable_hash
         @association.embed = :objects
         assert_equal({
-          title: 'Title 1', body: 'Body 1', 'comments' => [{ content: 'C1' }, { content: 'C2' }]
+          title: 'Title 1', body: 'Body 1', comments: [{ content: 'C1' }, { content: 'C2' }]
         }, @post_serializer.serializable_hash)
       end
 
       def test_associations_embedding_objects_serialization_using_as_json
         @association.embed = :objects
         assert_equal({
-          'post' => { title: 'Title 1', body: 'Body 1', 'comments' => [{ content: 'C1' }, { content: 'C2' }] }
+          'post' => { title: 'Title 1', body: 'Body 1', comments: [{ content: 'C1' }, { content: 'C2' }] }
         }, @post_serializer.as_json)
       end
 
@@ -63,7 +63,7 @@ module ActiveModel
         end
 
         assert_equal({
-          'post' => { title: 'Title 1', body: 'Body 1', 'comments' => [nil] }
+          'post' => { title: 'Title 1', body: 'Body 1', comments: [nil] }
         }, @post_serializer.as_json)
       end
 
@@ -109,7 +109,7 @@ module ActiveModel
 
         assert_equal({
           'post' => { title: 'Title 1', body: 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } },
-          'comments' => [{ content: 'fake' }, { content: 'fake' }]
+          comments: [{ content: 'fake' }, { content: 'fake' }]
         }, @post_serializer.as_json)
       end
     end

test/unit/active_model/serializer/has_one_test.rb

@@ -43,14 +43,14 @@ module ActiveModel
       def test_associations_embedding_objects_serialization_using_serializable_hash
         @association.embed = :objects
         assert_equal({
-          name: 'Name 1', email: 'mail@server.com', 'profiles' => [{ name: 'N1', description: 'D1' }]
+          name: 'Name 1', email: 'mail@server.com', profile: { name: 'N1', description: 'D1' }
         }, @user_serializer.serializable_hash)
       end
 
       def test_associations_embedding_objects_serialization_using_as_json
         @association.embed = :objects
         assert_equal({
-          'user' => { name: 'Name 1', email: 'mail@server.com', 'profiles' => [{ name: 'N1', description: 'D1' }] }
+          'user' => { name: 'Name 1', email: 'mail@server.com', profile: { name: 'N1', description: 'D1' } }
         }, @user_serializer.as_json)
       end
 
@@ -75,7 +75,7 @@ module ActiveModel
         end
 
         assert_equal({
-          'user' => { name: 'Name 1', email: 'mail@server.com', 'profiles' => [nil] }
+          'user' => { name: 'Name 1', email: 'mail@server.com', profile: nil }
         }, @user_serializer.as_json)
       end
 
@@ -83,7 +83,7 @@ module ActiveModel
         @association.embed = :objects
         @association.embedded_key = 'root'
         assert_equal({
-          name: 'Name 1', email: 'mail@server.com', 'root' => [{ name: 'N1', description: 'D1' }]
+          name: 'Name 1', email: 'mail@server.com', 'root' => { name: 'N1', description: 'D1' }
         }, @user_serializer.serializable_hash)
       end
 
@@ -100,7 +100,7 @@ module ActiveModel
         @user_serializer.root = nil
         assert_equal({
           'user' => { name: 'Name 1', email: 'mail@server.com', 'profile_id' => @user.profile.object_id },
-          'profiles' => [{ name: 'N1', description: 'D1' }]
+          profile: { name: 'N1', description: 'D1' }
         }, @user_serializer.as_json)
       end
 
@@ -117,7 +117,7 @@ module ActiveModel
 
         assert_equal({
           'user' => { name: 'Name 1', email: 'mail@server.com', 'profile_id' => @user.profile.object_id },
-          'profiles' => [{ name: 'fake' }]
+          profile: { name: 'fake' }
         }, @user_serializer.as_json)
       end
     end