diff --git a/lib/action_controller/serialization.rb b/lib/action_controller/serialization.rb
index 4ac32093..3dbbfbb2 100644
--- a/lib/action_controller/serialization.rb
+++ b/lib/action_controller/serialization.rb
@@ -64,15 +64,12 @@ module ActionController
     def build_json_serializer(resource, options)
       options = default_serializer_options.merge(options || {})
 
-      serializer = options.delete(:serializer)
-      serializer = ActiveModel::Serializer.serializer_for(resource) if serializer.nil?
+      if serializer = options.fetch(:serializer, ActiveModel::Serializer.serializer_for(resource))
+        options[:scope] = serialization_scope unless options.has_key?(:scope)
+        options[:resource_name] = self.controller_name if resource.respond_to?(:to_ary)
 
-      return unless serializer
-
-      options[:scope] = serialization_scope unless options.has_key?(:scope)
-      options[:resource_name] = self.controller_name if resource.respond_to?(:to_ary)
-
-      serializer.new(resource, options)
+        serializer.new(resource, options)
+      end
     end
   end
 end
diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index b3d55c1f..6bdb48e5 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -159,13 +159,7 @@ end
 
     def serialize(association)
       associated_data = send(association.name)
-      if associated_data.respond_to?(:to_ary) &&
-         !(association.serializer_class &&
-           association.serializer_class <= ArraySerializer)
-        associated_data.map { |elem| association.build_serializer(elem).serializable_hash }
-      else
-        association.build_serializer(associated_data).serializable_object
-      end
+      association.build_serializer(associated_data).serializable_object
     end
 
     def serialize_ids(association)
diff --git a/lib/active_model/serializer/associations.rb b/lib/active_model/serializer/associations.rb
index 2a5787c9..93319e76 100644
--- a/lib/active_model/serializer/associations.rb
+++ b/lib/active_model/serializer/associations.rb
@@ -17,7 +17,6 @@ module ActiveModel
         @embed_in_root = options.fetch(:embed_in_root) { options.fetch(:include) { CONFIG.embed_in_root } }
         @embed_key     = options[:embed_key] || :id
         @key           = options[:key]
-        @embedded_key  = options[:root] || name
 
         self.serializer_class = @options[:serializer]
       end
@@ -30,6 +29,13 @@ module ActiveModel
 
       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
+        else
+          @serializer_class ||= ArraySerializer
+        end
       end
 
       def embed=(embed)
@@ -38,21 +44,22 @@ module ActiveModel
       end
 
       def build_serializer(object)
-        @serializer_class ||= Serializer.serializer_for(object) || DefaultSerializer
-        @serializer_class.new(object, @options)
+        @serializer_class.new(Array(object), @options)
       end
 
       class HasOne < Association
-        def initialize(*args)
+        def initialize(name, *args)
           super
-          @key  ||= "#{name}_id"
+          @embedded_key = "#{@options[:root] || name}".pluralize
+          @key ||= "#{name}_id"
         end
       end
 
       class HasMany < Association
-        def initialize(*args)
+        def initialize(name, *args)
           super
-          @key ||= "#{name.singularize}_ids"
+          @embedded_key = @options[:root] || name
+          @key ||= "#{name.to_s.singularize}_ids"
         end
       end
     end
diff --git a/test/integration/active_record/active_record_test.rb b/test/integration/active_record/active_record_test.rb
index 0bad4672..8e42d446 100644
--- a/test/integration/active_record/active_record_test.rb
+++ b/test/integration/active_record/active_record_test.rb
@@ -17,7 +17,7 @@ module ActiveModel
             ar_comments: [{ body: 'what a dumb post', ar_tags: [{ name: 'short' }, { name: 'whiny' }] },
                           { body: 'i liked it', ar_tags: [{:name=>"short"}, {:name=>"happy"}] }],
             ar_tags: [{ name: 'short' }, { name: 'whiny' }, { name: 'happy' }],
-            ar_section: { 'name' => 'ruby' }
+            'ar_sections' => [{ 'name' => 'ruby' }]
           }
         }, post_serializer.as_json)
       end
@@ -51,7 +51,7 @@ module ActiveModel
             ar_comments: [{ body: 'what a dumb post', ar_tags: [{ name: 'short' }, { name: 'whiny' }] },
                           { body: 'i liked it', ar_tags: [{:name=>"short"}, {:name=>"happy"}] }],
             ar_tags: [{ name: 'short' }, { name: 'whiny' }, { name: 'happy' }],
-            ar_section: { 'name' => 'ruby' }
+            'ar_sections' => [{ 'name' => 'ruby' }]
           }, post_serializer.as_json)
         end
       end
diff --git a/test/unit/active_model/serializer/has_one_test.rb b/test/unit/active_model/serializer/has_one_test.rb
index 3dd72693..b544670f 100644
--- a/test/unit/active_model/serializer/has_one_test.rb
+++ b/test/unit/active_model/serializer/has_one_test.rb
@@ -50,7 +50,7 @@ module ActiveModel
         @association.embed = :objects
 
         assert_equal({
-          name: 'Name 1', email: 'mail@server.com', profile: { name: 'N1', description: 'D1' }
+          name: 'Name 1', email: 'mail@server.com', 'profiles' => [{ name: 'N1', description: 'D1' }]
         }, @user_serializer.serializable_hash)
       end
 
@@ -58,7 +58,7 @@ module ActiveModel
         @association.embed = :objects
 
         assert_equal({
-          'user' => { name: 'Name 1', email: 'mail@server.com', profile: { name: 'N1', description: 'D1' } }
+          'user' => { name: 'Name 1', email: 'mail@server.com', 'profiles' => [{ name: 'N1', description: 'D1' }] }
         }, @user_serializer.as_json)
       end
 
@@ -84,7 +84,7 @@ module ActiveModel
         end
 
         assert_equal({
-          'user' => { name: 'Name 1', email: 'mail@server.com', profile: nil }
+          'user' => { name: 'Name 1', email: 'mail@server.com', 'profiles' => [] }
         }, @user_serializer.as_json)
       end
 
@@ -93,7 +93,7 @@ module ActiveModel
         @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
 
@@ -112,7 +112,7 @@ module ActiveModel
 
         assert_equal({
           'user' => { name: 'Name 1', email: 'mail@server.com', 'profile_id' => @user.profile.object_id },
-          profile: { name: 'N1', description: 'D1' }
+          'profiles' => [{ name: 'N1', description: 'D1' }]
         }, @user_serializer.as_json)
       end
 
@@ -129,7 +129,7 @@ module ActiveModel
 
         assert_equal({
           'user' => { name: 'Name 1', email: 'mail@server.com', 'profile_id' => @user.profile.object_id },
-          profile: { name: 'fake' }
+          'profiles' => [{ name: 'fake' }]
         }, @user_serializer.as_json)
       end
     end