diff --git a/lib/action_controller/serialization.rb b/lib/action_controller/serialization.rb
index 6fcb5df7..415167ef 100644
--- a/lib/action_controller/serialization.rb
+++ b/lib/action_controller/serialization.rb
@@ -6,7 +6,7 @@ module ActionController
 
     include ActionController::Renderers
 
-    ADAPTER_OPTION_KEYS = [:include, :fields, :root, :adapter]
+    ADAPTER_OPTION_KEYS = [:include, :fields, :adapter]
 
     included do
       class_attribute :_serialization_scope
diff --git a/lib/active_model/serializer/adapter.rb b/lib/active_model/serializer/adapter.rb
index 9c95f5ee..0cbab9d3 100644
--- a/lib/active_model/serializer/adapter.rb
+++ b/lib/active_model/serializer/adapter.rb
@@ -22,7 +22,8 @@ module ActiveModel
 
       def as_json(options = {})
         hash = serializable_hash(options)
-        include_meta(hash)
+        include_meta(hash) unless self.class == FlattenJson
+        hash
       end
 
       def self.create(resource, options = {})
diff --git a/lib/active_model/serializer/array_serializer.rb b/lib/active_model/serializer/array_serializer.rb
index f88a101a..b05cb1dd 100644
--- a/lib/active_model/serializer/array_serializer.rb
+++ b/lib/active_model/serializer/array_serializer.rb
@@ -7,8 +7,6 @@ module ActiveModel
       attr_reader :meta, :meta_key
 
       def initialize(objects, options = {})
-        options.merge!(root: nil)
-
         @objects = objects.map do |object|
           serializer_class = options.fetch(
             :serializer,
@@ -21,7 +19,7 @@ module ActiveModel
       end
 
       def json_key
-        @objects.first.json_key.pluralize if @objects.first
+        @objects.first.json_key if @objects.first
       end
 
       def root=(root)
diff --git a/lib/active_model/serializer/fieldset.rb b/lib/active_model/serializer/fieldset.rb
index a09fad18..911dac25 100644
--- a/lib/active_model/serializer/fieldset.rb
+++ b/lib/active_model/serializer/fieldset.rb
@@ -12,7 +12,7 @@ module ActiveModel
       end
 
       def fields_for(serializer)
-        key = serializer.json_key || serializer.class.root_name
+        key = serializer.json_key
         fields[key.to_sym]
       end
 
diff --git a/test/action_controller/explicit_serializer_test.rb b/test/action_controller/explicit_serializer_test.rb
index d4cbdb4d..126db5c7 100644
--- a/test/action_controller/explicit_serializer_test.rb
+++ b/test/action_controller/explicit_serializer_test.rb
@@ -77,12 +77,10 @@ module ActionController
         get :render_array_using_explicit_serializer
         assert_equal 'application/json', @response.content_type
 
-        expected = {
-          'paginated' => [
-            { 'name' => 'Name 1' },
-            { 'name' => 'Name 2' }
-          ]
-        }
+        expected = [
+          { 'name' => 'Name 1' },
+          { 'name' => 'Name 2' }
+        ]
 
         assert_equal expected.to_json, @response.body
       end
diff --git a/test/action_controller/serialization_test.rb b/test/action_controller/serialization_test.rb
index ff646a97..5fc4c529 100644
--- a/test/action_controller/serialization_test.rb
+++ b/test/action_controller/serialization_test.rb
@@ -10,13 +10,17 @@ module ActionController
         end
 
         def render_using_custom_root
-          @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
-          render json: @profile, root: "custom_root"
+          with_adapter ActiveModel::Serializer::Adapter::Json do
+            @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
+            render json: @profile, root: "custom_root"
+          end
         end
 
         def render_using_custom_root_and_meta
-          @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
-          render json: @profile, root: "custom_root", meta: { total: 10 }
+          with_adapter ActiveModel::Serializer::Adapter::Json do
+            @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
+            render json: @profile, root: "custom_root", meta: { total: 10 }
+          end
         end
 
         def render_using_default_adapter_root
@@ -34,11 +38,13 @@ module ActionController
         end
 
         def render_array_using_custom_root_and_meta
-          array = [
-            Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' }),
-            Profile.new({ name: 'Name 2', description: 'Description 2', comments: 'Comments 2' })
-          ]
-          render json: array, root: "custom_root", meta: { total: 10 }
+          with_adapter ActiveModel::Serializer::Adapter::Json do
+            array = [
+              Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' }),
+              Profile.new({ name: 'Name 2', description: 'Description 2', comments: 'Comments 2' })
+            ]
+            render json: array, root: "custom_root", meta: { total: 10 }
+          end
         end
 
         def render_array_using_implicit_serializer
@@ -219,6 +225,7 @@ module ActionController
 
       def test_render_array_using_custom_root_and_meta
         get :render_array_using_custom_root_and_meta
+
         assert_equal 'application/json', @response.content_type
 
         expected = { custom_root: [
diff --git a/test/adapter/json/belongs_to_test.rb b/test/adapter/json/belongs_to_test.rb
index 1252fe38..1d20d1c9 100644
--- a/test/adapter/json/belongs_to_test.rb
+++ b/test/adapter/json/belongs_to_test.rb
@@ -25,21 +25,21 @@ module ActiveModel
           end
 
           def test_includes_post
-            assert_equal({id: 42, title: 'New Post', body: 'Body'}, @adapter.serializable_hash[:post])
+            assert_equal({id: 42, title: 'New Post', body: 'Body'}, @adapter.serializable_hash[:comment][:post])
           end
 
           def test_include_nil_author
             serializer = PostSerializer.new(@anonymous_post)
             adapter = ActiveModel::Serializer::Adapter::Json.new(serializer)
 
-            assert_equal({title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], blog: {id: 999, name: "Custom blog"}, author: nil}, adapter.serializable_hash)
+            assert_equal({post: {title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], blog: {id: 999, name: "Custom blog"}, author: nil}}, adapter.serializable_hash)
           end
 
           def test_include_nil_author_with_specified_serializer
             serializer = PostPreviewSerializer.new(@anonymous_post)
             adapter = ActiveModel::Serializer::Adapter::Json.new(serializer)
 
-            assert_equal({title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], author: nil}, adapter.serializable_hash)
+            assert_equal({posts: {title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], author: nil}}, adapter.serializable_hash)
           end
         end
       end
diff --git a/test/adapter/json/collection_test.rb b/test/adapter/json/collection_test.rb
index d91fb2b6..018a6b23 100644
--- a/test/adapter/json/collection_test.rb
+++ b/test/adapter/json/collection_test.rb
@@ -28,16 +28,16 @@ module ActiveModel
             @serializer = ArraySerializer.new([@blog], serializer: CustomBlogSerializer)
             @adapter = ActiveModel::Serializer::Adapter::Json.new(@serializer)
 
-            expected = [{
+            expected = {custom_blog:[{
               id: 1,
               special_attribute: "Special",
               articles: [{id: 1,title: "Hello!!", body: "Hello, world!!"}, {id: 2, title: "New Post", body: "Body"}]
-            }]
+            }]}
             assert_equal expected, @adapter.serializable_hash
           end
 
           def test_include_multiple_posts
-            expected = [{
+            expected = { post: [{
               title: "Hello!!",
               body: "Hello, world!!",
               id: 1,
@@ -63,7 +63,7 @@ module ActiveModel
                 id: 999,
                 name: "Custom blog"
               }
-            }]
+            }]}
             assert_equal expected, @adapter.serializable_hash
           end
         end
diff --git a/test/adapter/json/has_many_test.rb b/test/adapter/json/has_many_test.rb
index b73af9f5..19fe16cd 100644
--- a/test/adapter/json/has_many_test.rb
+++ b/test/adapter/json/has_many_test.rb
@@ -26,7 +26,7 @@ module ActiveModel
             assert_equal([
                            {id: 1, body: 'ZOMG A COMMENT'},
                            {id: 2, body: 'ZOMG ANOTHER COMMENT'}
-                         ], @adapter.serializable_hash[:comments])
+                         ], @adapter.serializable_hash[:post][:comments])
           end
         end
       end
diff --git a/test/adapter/json_api/collection_test.rb b/test/adapter/json_api/collection_test.rb
index f17285fa..f0d4eff1 100644
--- a/test/adapter/json_api/collection_test.rb
+++ b/test/adapter/json_api/collection_test.rb
@@ -86,7 +86,6 @@ module ActiveModel
                 }
               }
             ]
-
             assert_equal(expected, @adapter.serializable_hash[:data])
           end
 
diff --git a/test/adapter/json_test.rb b/test/adapter/json_test.rb
index 52c9d8fb..efc4c834 100644
--- a/test/adapter/json_test.rb
+++ b/test/adapter/json_test.rb
@@ -25,7 +25,7 @@ module ActiveModel
           assert_equal([
                          {id: 1, body: 'ZOMG A COMMENT'},
                          {id: 2, body: 'ZOMG ANOTHER COMMENT'}
-                       ], @adapter.serializable_hash[:comments])
+                       ], @adapter.serializable_hash[:post][:comments])
         end
       end
     end
diff --git a/test/adapter_test.rb b/test/adapter_test.rb
index d76559d5..7268c386 100644
--- a/test/adapter_test.rb
+++ b/test/adapter_test.rb
@@ -31,7 +31,7 @@ module ActiveModel
 
       def test_create_adapter
         adapter = ActiveModel::Serializer::Adapter.create(@serializer)
-        assert_equal ActiveModel::Serializer::Adapter::Json, adapter.class
+        assert_equal ActiveModel::Serializer::Adapter::FlattenJson, adapter.class
       end
 
       def test_create_adapter_with_override
diff --git a/test/serializers/adapter_for_test.rb b/test/serializers/adapter_for_test.rb
index 60641080..507b6bf1 100644
--- a/test/serializers/adapter_for_test.rb
+++ b/test/serializers/adapter_for_test.rb
@@ -11,7 +11,7 @@ module ActiveModel
 
       def test_returns_default_adapter
         adapter = ActiveModel::Serializer.adapter
-        assert_equal ActiveModel::Serializer::Adapter::Json, adapter
+        assert_equal ActiveModel::Serializer::Adapter::FlattenJson, adapter
       end
 
       def test_overwrite_adapter_with_symbol
diff --git a/test/serializers/attribute_test.rb b/test/serializers/attribute_test.rb
index b75df48c..1b52216a 100644
--- a/test/serializers/attribute_test.rb
+++ b/test/serializers/attribute_test.rb
@@ -15,13 +15,13 @@ module ActiveModel
 
       def test_json_serializable_hash
         adapter = ActiveModel::Serializer::Adapter::Json.new(@blog_serializer)
-        assert_equal({:id=>1, :title=>"AMS Hints"}, adapter.serializable_hash)
+        assert_equal({alternate_blog: { id:1, title:"AMS Hints"}}, adapter.serializable_hash)
       end
 
       def test_attribute_inheritance_with_key
         inherited_klass = Class.new(AlternateBlogSerializer)
         blog_serializer = inherited_klass.new(@blog)
-        adapter = ActiveModel::Serializer::Adapter::Json.new(blog_serializer)
+        adapter = ActiveModel::Serializer::Adapter::FlattenJson.new(blog_serializer)
         assert_equal({:id=>1, :title=>"AMS Hints"}, adapter.serializable_hash)
       end
 
diff --git a/test/serializers/configuration_test.rb b/test/serializers/configuration_test.rb
index 9c6c5fea..e623cde9 100644
--- a/test/serializers/configuration_test.rb
+++ b/test/serializers/configuration_test.rb
@@ -8,7 +8,7 @@ module ActiveModel
       end
 
       def test_default_adapter
-        assert_equal :json, ActiveModel::Serializer.config.adapter
+        assert_equal :flatten_json, ActiveModel::Serializer.config.adapter
       end
     end
   end
diff --git a/test/serializers/meta_test.rb b/test/serializers/meta_test.rb
index 2ec90611..b6201574 100644
--- a/test/serializers/meta_test.rb
+++ b/test/serializers/meta_test.rb
@@ -12,9 +12,10 @@ module ActiveModel
       end
 
       def test_meta_is_present_with_root
-        adapter = load_adapter(root: "blog", meta: {total: 10})
+        serializer = AlternateBlogSerializer.new(@blog, root: "blog", meta: {total: 10})
+        adapter = ActiveModel::Serializer::Adapter::Json.new(serializer, root: 'blog')
         expected = {
-          "blog" => {
+          blog: {
             id: 1,
             title: "AMS Hints"
           },
@@ -35,9 +36,10 @@ module ActiveModel
       end
 
       def test_meta_key_is_used
-        adapter = load_adapter(root: "blog", meta: {total: 10}, meta_key: "haha_meta")
+        serializer = AlternateBlogSerializer.new(@blog, root: 'blog', meta: {total: 10}, meta_key: "haha_meta")
+        adapter = ActiveModel::Serializer::Adapter::Json.new(serializer, root: 'blog')
         expected = {
-          "blog" => {
+          blog: {
             id: 1,
             title: "AMS Hints"
           },
@@ -50,7 +52,7 @@ module ActiveModel
 
       def test_meta_is_not_present_on_arrays_without_root
         serializer = ArraySerializer.new([@blog], meta: {total: 10})
-        adapter = ActiveModel::Serializer::Adapter::Json.new(serializer)
+        adapter = ActiveModel::Serializer::Adapter::FlattenJson.new(serializer)
         expected = [{
           id: 1,
           name: "AMS Hints",
@@ -71,7 +73,7 @@ module ActiveModel
         serializer = ArraySerializer.new([@blog], meta: {total: 10}, meta_key: "haha_meta")
         adapter = ActiveModel::Serializer::Adapter::Json.new(serializer, root: 'blog')
         expected = {
-          'blog' => [{
+          blog: [{
             id: 1,
             name: "AMS Hints",
             writer: {
@@ -98,7 +100,7 @@ module ActiveModel
           options.partition { |k, _| ActionController::Serialization::ADAPTER_OPTION_KEYS.include? k }.map { |h| Hash[h] }
 
         serializer = AlternateBlogSerializer.new(@blog, serializer_opts)
-        ActiveModel::Serializer::Adapter::Json.new(serializer, adapter_opts)
+        ActiveModel::Serializer::Adapter::FlattenJson.new(serializer, adapter_opts)
       end
     end
   end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index a8b48355..be06c505 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -20,6 +20,7 @@ class Foo < Rails::Application
     config.active_support.test_order = :random
     config.logger = Logger.new(nil)
     ActionController::Base.cache_store = :memory_store
+    ActiveModel::Serializer.config.adapter = :flatten_json
   end
 end
 FileUtils.mkdir_p(File.expand_path('../../tmp/cache', __FILE__))