diff --git a/lib/active_model_serializers/model.rb b/lib/active_model_serializers/model.rb
index 8de5a27d1..acc7a4a82 100644
--- a/lib/active_model_serializers/model.rb
+++ b/lib/active_model_serializers/model.rb
@@ -9,22 +9,7 @@ class Model
     # Configuration to avoid a breaking change with older versions of this class which lacked defined attributes.
     # Previous behavior was: 1) initialized attributes were the
     class_attribute :attributes_are_always_the_initialization_data, instance_writer: false, instance_reader: false
-    self.attributes_are_always_the_initialization_data = false # remove this commit, just for demonstration
-    module AttributesAreAlwaysTheInitializationData
-      def initialize(attributes = {})
-        @initialized_attributes = attributes && attributes.symbolize_keys
-        super
-      end
-
-      # Defaults to the downcased model name.
-      def id
-        @initialized_attributes.fetch(:id) { self.class.model_name.name && self.class.model_name.name.downcase }
-      end
-
-      def attributes
-        @initialized_attributes
-      end
-    end
+    self.attributes_are_always_the_initialization_data = true
 
     def self.inherited(subclass)
       if subclass.attributes_are_always_the_initialization_data
@@ -96,5 +81,21 @@ def self.lookup_ancestors
       [self]
     end
     # :nocov:
+
+    module AttributesAreAlwaysTheInitializationData
+      def initialize(attributes = {})
+        @initialized_attributes = attributes && attributes.symbolize_keys
+        super
+      end
+
+      # Defaults to the downcased model name.
+      def id
+        @initialized_attributes.fetch(:id) { self.class.model_name.name && self.class.model_name.name.downcase }
+      end
+
+      def attributes
+        @initialized_attributes
+      end
+    end
   end
 end
diff --git a/test/action_controller/adapter_selector_test.rb b/test/action_controller/adapter_selector_test.rb
index 6f22aae25..7489598a5 100644
--- a/test/action_controller/adapter_selector_test.rb
+++ b/test/action_controller/adapter_selector_test.rb
@@ -3,6 +3,14 @@
 module ActionController
   module Serialization
     class AdapterSelectorTest < ActionController::TestCase
+      Profile = poro_without_legacy_model_support(::Model) do
+        attributes :name, :description
+        associations :comments
+      end
+      class ProfileSerializer < ActiveModel::Serializer
+        type 'profiles'
+        attributes :name, :description
+      end
       class AdapterSelectorTestController < ActionController::Base
         def render_using_default_adapter
           @profile = Profile.new(name: 'Name 1', description: 'Description 1', comments: 'Comments 1')
diff --git a/test/active_model_serializers/model_test.rb b/test/active_model_serializers/model_test.rb
index c2f316479..4435f8192 100644
--- a/test/active_model_serializers/model_test.rb
+++ b/test/active_model_serializers/model_test.rb
@@ -20,7 +20,7 @@ def test_initialization_with_string_keys
     end
 
     def test_attributes_can_be_read_for_serialization
-      klass = Class.new(ActiveModelSerializers::Model) do
+      klass = poro_without_legacy_model_support do
         attributes :one, :two, :three
       end
       original_attributes = { one: 1, two: 2, three: 3 }
@@ -42,7 +42,7 @@ def test_attributes_can_be_read_for_serialization
     end
 
     def test_id_attribute_can_be_read_for_serialization
-      klass = Class.new(ActiveModelSerializers::Model) do
+      klass = poro_without_legacy_model_support do
         attributes :id, :one, :two, :three
       end
       self.class.const_set(:SomeTestModel, klass)
diff --git a/test/cache_test.rb b/test/cache_test.rb
index b2cb27eb4..9c00c4ed9 100644
--- a/test/cache_test.rb
+++ b/test/cache_test.rb
@@ -49,7 +49,7 @@ class InheritedRoleSerializer < RoleSerializer
       attribute :special_attribute
     end
 
-    class Comment < ::Model
+    Comment = poro_without_legacy_model_support(::Model) do
       attributes :body
       associations :post, :author
 
diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index 3c804ccca..6abecef74 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -1,3 +1,4 @@
+ActiveModelSerializers::Model.attributes_are_always_the_initialization_data = false
 class Model < ActiveModelSerializers::Model
   FILE_DIGEST = Digest::MD5.hexdigest(File.open(__FILE__).read)
 
diff --git a/test/support/poro.rb b/test/support/poro.rb
new file mode 100644
index 000000000..e7f525b11
--- /dev/null
+++ b/test/support/poro.rb
@@ -0,0 +1,15 @@
+module ActiveModelSerializersWithoutLegacyModelSupport
+  module_function
+
+  def poro_without_legacy_model_support(superklass = ActiveModelSerializers::Model, &block)
+    original_attributes_are_always_the_initialization_data = superklass.attributes_are_always_the_initialization_data
+    superklass.attributes_are_always_the_initialization_data = false
+    Class.new(superklass) do
+      class_exec(&block) if block
+    end
+  ensure
+    superklass.attributes_are_always_the_initialization_data = original_attributes_are_always_the_initialization_data
+  end
+end
+Minitest::Test.include ActiveModelSerializersWithoutLegacyModelSupport
+Minitest::Test.extend ActiveModelSerializersWithoutLegacyModelSupport
diff --git a/test/test_helper.rb b/test/test_helper.rb
index e96c4840f..664374488 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -40,6 +40,8 @@ def serialization_options(options)
 require 'minitest/autorun'
 Minitest.backtrace_filter = Minitest::BacktraceFilter.new
 
+require 'support/poro'
+
 require 'support/rails_app'
 
 # require "rails/test_help"