diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1551b2cf7..cf0203dc7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,29 +14,26 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Recent Rubies and Rails
-          # NOTE(BF): Rails >= 5.0 has test failures which need to be fixed
-          # - ruby-version: '2.6'
-          #   rails-version: '6.1'
-          # - ruby-version: '2.7'
-          #   rails-version: '6.0'
-          # - ruby-version: '2.6'
-          #   rails-version: '5.2'
-          # - ruby-version: '2.5'
-          #   rails-version: '5.1'
-          #   bundler: '1'
-          # - ruby-version: '2.4'
-          #   rails-version: '5.0'
-          #   bundler: '1'
-          - ruby-version: '2.4'
-            rails-version: '4.2'
-            bundler: '1'
-          - ruby-version: '2.4'
-            rails-version: '4.1'
-            bundler: '1'
-          - ruby-version: '2.4'
-            rails-version: '4.0'
-            bundler: '1'
+          - ruby-version: '2.7'
+            rails-version: '5.0'
+          - ruby-version: '2.7'
+            rails-version: '5.1'
+          - ruby-version: '2.7'
+            rails-version: '5.2'
+          - ruby-version: '3.0'
+            rails-version: '6.0'
+          - ruby-version: '3.1'
+            rails-version: '6.1'
+          - ruby-version: '3.2'
+            rails-version: '7.0'
+          - ruby-version: '3.3'
+            rails-version: '7.1'
+          - ruby-version: '3.3'
+            rails-version: '7.2'
+          - ruby-version: '3.4'
+            rails-version: '8.0'
+          - ruby-version: '4.0'
+            rails-version: '8.1'
 
     continue-on-error: "${{ endsWith(matrix.ruby-version, 'head') }}"
 
@@ -44,17 +41,16 @@ jobs:
       CI: "1"
 
     steps:
-      - name:                     "Checkout Code"
-        uses:                     "actions/checkout@v2"
-        timeout-minutes:          5
+      - name: "Checkout Code"
+        uses: "actions/checkout@v2"
+        timeout-minutes: 5
         with:
-          fetch-depth:            0
+          fetch-depth: 0
 
       - name: "Build Ruby"
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version:   "${{ matrix.ruby-version }}"
-          bundler:        "${{ matrix.bundler || 2 }}"
+          ruby-version: "${{ matrix.ruby-version }}"
           bundler-cache:  true
         env:
           RAILS_VERSION: ${{ matrix.rails-version }}
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index a391e3ddf..000000000
--- a/.travis.yml
+++ /dev/null
@@ -1,35 +0,0 @@
-language: ruby
-sudo: false
-rvm:
-  # - 1.8.7
-  # - ree
-  # - jruby-18mode
-  # - 1.9.2
-  # - 1.9.3
-  - jruby-19mode
-  - 2.0.0
-  - 2.1
-  - 2.2
-install: bundle install --path=vendor/bundle --retry=3 --jobs=3
-cache:
-  directories:
-    - vendor/bundle
-script:
-  - bundle exec rake ci
-env:
-  # - "RAILS_VERSION=3.0"
-  # - "RAILS_VERSION=3.1"
-  # - "RAILS_VERSION=3.2"
-  - "RAILS_VERSION=4.0"
-  - "RAILS_VERSION=4.1"
-  - "RAILS_VERSION=4.2"
-  # - "RAILS_VERSION=master"
-matrix:
-  exclude:
-    - rvm: jruby-19mode
-      env: "RAILS_VERSION=4.0"
-  # - rvm: 1.8.7
-  # - ree
-  # - jruby-18mode
-  # - rvm: 1.9.2
-  fast_finish: true
diff --git a/Gemfile b/Gemfile
index 384d7c261..cefff2346 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,40 +1,15 @@
 source 'https://rubygems.org'
 
-version = ENV['RAILS_VERSION'] || '4.1'
-
+version = ENV['RAILS_VERSION']
 if version == 'master'
-  gem 'rack', github: 'rack/rack'
-  gem 'arel', github: 'rails/arel'
   gem 'rails', github: 'rails/rails'
-  git 'https://github.com/rails/rails.git' do
-    gem 'railties'
-    gem 'activesupport'
-    gem 'activemodel'
-    gem 'actionpack'
-    gem 'activerecord', group: :test
-    # Rails 5
-    gem 'actionview'
-  end
-else
+elsif version
   gem_version = "~> #{version}.0"
   gem 'rails', gem_version
-  gem 'railties', gem_version
-  gem 'activesupport', gem_version
-  gem 'activemodel', gem_version
-  gem 'actionpack', gem_version
-  gem 'activerecord', gem_version, group: :test
+else
+  gem 'rails'
 end
 
-# Specify gem dependencies in active_model_serializers.gemspec
 gemspec
 
-# https://github.com/bundler/bundler/blob/89a8778c19269561926cea172acdcda241d26d23/lib/bundler/dependency.rb#L30-L54
-@windows_platforms = [:mswin, :mswin64, :mingw, :x64_mingw]
-
-# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
-gem 'tzinfo-data', platforms: (@windows_platforms + [:jruby])
-
-# JRuby versions before 9.x report their version as "1.9.x" or lower, so lock these to an older version of mime-types
-if defined?(JRUBY_VERSION) and Gem::ruby_version < Gem::Version.new("2.0.0")
-  gem 'mime-types', '< 3'
-end
+gem 'tzinfo-data', platforms: %i[mswin mswin64 mingw x64_mingw jruby]
diff --git a/active_model_serializers.gemspec b/active_model_serializers.gemspec
index c5724df99..c4c87d423 100644
--- a/active_model_serializers.gemspec
+++ b/active_model_serializers.gemspec
@@ -16,9 +16,10 @@ Gem::Specification.new do |gem|
   gem.name          = "active_model_serializers"
   gem.require_paths = ["lib"]
   gem.version       = ActiveModel::Serializer::VERSION
+  gem.required_ruby_version = '>= 2.7'
 
-  gem.add_dependency 'activemodel', '>= 3.0'
-  gem.add_development_dependency "rails", ">= 3.0"
+  gem.add_dependency 'activemodel', '>= 6.0'
+  gem.add_development_dependency "rails", ">= 6.0"
   gem.add_development_dependency "pry"
   gem.add_development_dependency "minitest"
 end
diff --git a/bench/perf.rb b/bench/perf.rb
index ea668d56e..836a0785a 100644
--- a/bench/perf.rb
+++ b/bench/perf.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rubygems"
 require "bundler/setup"
 require "active_model_serializers"
diff --git a/lib/action_controller/serialization.rb b/lib/action_controller/serialization.rb
index 4bcad5667..add6aa396 100644
--- a/lib/action_controller/serialization.rb
+++ b/lib/action_controller/serialization.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActionController
   # Action Controller Serialization
   #
diff --git a/lib/active_model/array_serializer.rb b/lib/active_model/array_serializer.rb
index fc3159ad7..1fbe488b8 100644
--- a/lib/active_model/array_serializer.rb
+++ b/lib/active_model/array_serializer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "active_support/core_ext/class/attribute"
 require 'active_support/dependencies'
 require 'active_support/descendants_tracker'
diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 694cf61f7..0cbc7409a 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "active_support/core_ext/class/attribute"
 require "active_support/core_ext/module/anonymous"
 require 'active_support/dependencies'
@@ -455,7 +457,7 @@ def merge_association(hash, key, serializables, unique_values)
     def attributes
       _fast_attributes
       rescue NameError
-        method = "def _fast_attributes\n"
+        method = +"def _fast_attributes\n"
 
         method << "  h = {}\n"
 
diff --git a/lib/active_model/serializer/associations.rb b/lib/active_model/serializer/associations.rb
index 8606b930e..5ec65c511 100644
--- a/lib/active_model/serializer/associations.rb
+++ b/lib/active_model/serializer/associations.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveModel
   class Serializer
     module Associations #:nodoc:
diff --git a/lib/active_model/serializers/version.rb b/lib/active_model/serializers/version.rb
index 96a2681b3..08a46c449 100644
--- a/lib/active_model/serializers/version.rb
+++ b/lib/active_model/serializers/version.rb
@@ -1,5 +1,7 @@
+# frozen_string_literal: true
+
 module ActiveModel
   class Serializer
-    VERSION = "0.8.4"
+    VERSION = "0.8.5"
   end
 end
diff --git a/lib/active_model_serializers.rb b/lib/active_model_serializers.rb
index 26a3353e7..499ff04c7 100644
--- a/lib/active_model_serializers.rb
+++ b/lib/active_model_serializers.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "active_support"
 require "active_support/core_ext/string/inflections"
 require "active_support/notifications"
diff --git a/lib/active_record/serializer_override.rb b/lib/active_record/serializer_override.rb
index b6149b83a..1df04c046 100644
--- a/lib/active_record/serializer_override.rb
+++ b/lib/active_record/serializer_override.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # We do not recommend that you use AM::S in this way, but if you must, here
 # is a mixin that overrides ActiveRecord::Base#to_json and #as_json.
 
diff --git a/lib/generators/resource_override.rb b/lib/generators/resource_override.rb
index 1b48a12e7..dd8867fea 100644
--- a/lib/generators/resource_override.rb
+++ b/lib/generators/resource_override.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rails/generators"
 require "rails/generators/rails/resource/resource_generator"
 
diff --git a/lib/generators/serializer/serializer_generator.rb b/lib/generators/serializer/serializer_generator.rb
index b7096f835..574f8191d 100644
--- a/lib/generators/serializer/serializer_generator.rb
+++ b/lib/generators/serializer/serializer_generator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Rails
   module Generators
     class SerializerGenerator < NamedBase
diff --git a/lib/generators/serializer/templates/serializer.rb b/lib/generators/serializer/templates/serializer.rb
index 6c25609d4..61bc9ada4 100644
--- a/lib/generators/serializer/templates/serializer.rb
+++ b/lib/generators/serializer/templates/serializer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 <% module_namespacing do -%>
 class <%= class_name %>Serializer < <%= parent_class_name %>
   attributes <%= attributes_names.map(&:inspect).join(", ") %>
diff --git a/test/array_serializer_test.rb b/test/array_serializer_test.rb
index fd9ffbb5f..240aff2f0 100644
--- a/test/array_serializer_test.rb
+++ b/test/array_serializer_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "test_helper"
 require "test_fakes"
 
@@ -17,6 +19,7 @@ def test_array_items_do_not_have_root
     each_serializer = array.active_model_serializer.new(array, :root => "root", :each_serializer => BasicSerializer)
 
     default_json = default_serializer.as_json
+    default_json['root'].map!(&:symbolize_keys)
     each_json = each_serializer.as_json
 
     assert_equal(expected, default_json)
diff --git a/test/association_test.rb b/test/association_test.rb
index c4eb121af..5d1414154 100644
--- a/test/association_test.rb
+++ b/test/association_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "test_helper"
 
 class AssociationTest < ActiveSupport::TestCase
@@ -367,11 +369,9 @@ def test_embed_ids_include_true_does_not_serialize_multiple_times
 
       # Count how often the @comment record is serialized.
       serialized_times = 0
-      @comment.class_eval do
-        define_method :read_attribute_for_serialization, lambda { |name|
-          serialized_times += 1 if name == :body
-          super(name)
-        }
+      @comment.define_singleton_method(:read_attribute_for_serialization) do |name|
+        serialized_times += 1 if name == :body
+        super(name)
       end
 
       include_bare! :comment
@@ -386,14 +386,12 @@ def test_include_with_read_association_id_for_serialization_hook
       end
 
       association_name = nil
-      @post.class_eval do
-        define_method :read_attribute_for_serialization, lambda { |name|
-          association_name = name
-          send(name)
-        }
-        define_method :comment_id, lambda {
-          @attributes[:comment].id
-        }
+      @post.define_singleton_method(:read_attribute_for_serialization) do |name|
+        association_name = name
+        send(name)
+      end
+      @post.define_singleton_method(:comment_id) do
+        @attributes[:comment].id
       end
 
       include_bare! :comment
@@ -409,14 +407,12 @@ def test_include_with_read_association_ids_for_serialization_hook
       end
 
       association_name = nil
-      @post.class_eval do
-        define_method :read_attribute_for_serialization, lambda { |name|
-          association_name = name
-          send(name)
-        }
-        define_method :comment_ids, lambda {
-          @attributes[:comments].map(&:id)
-        }
+      @post.define_singleton_method(:read_attribute_for_serialization) do |name|
+        association_name = name
+        send(name)
+      end
+      @post.define_singleton_method(:comment_ids) do
+        @attributes[:comments].map(&:id)
       end
 
       include_bare! :comments
@@ -478,7 +474,7 @@ def test_mutual_relation_result
     end
 
     def test_mutual_relation_does_not_raise_error
-      assert_nothing_raised SystemStackError, 'stack level too deep' do
+      assert_nothing_raised do # no SystemStackError
         @serializer.as_json
       end
     end
diff --git a/test/caching_test.rb b/test/caching_test.rb
index d66f2e34e..b8f582ad8 100644
--- a/test/caching_test.rb
+++ b/test/caching_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "test_helper"
 
 class CachingTest < ActiveSupport::TestCase
diff --git a/test/generators_test.rb b/test/generators_test.rb
index b1938cee8..4a3bda54b 100644
--- a/test/generators_test.rb
+++ b/test/generators_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class Foo < Rails::Application
diff --git a/test/no_serialization_scope_test.rb b/test/no_serialization_scope_test.rb
index 719ce4bfa..65b6601e6 100644
--- a/test/no_serialization_scope_test.rb
+++ b/test/no_serialization_scope_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "test_helper"
 
 class NoSerializationScopeTest < ActionController::TestCase
diff --git a/test/serialization_scope_name_test.rb b/test/serialization_scope_name_test.rb
index e419c6310..011540936 100644
--- a/test/serialization_scope_name_test.rb
+++ b/test/serialization_scope_name_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 require 'pathname'
 
diff --git a/test/serialization_test.rb b/test/serialization_test.rb
index 1465585df..d46cf6494 100644
--- a/test/serialization_test.rb
+++ b/test/serialization_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 require 'pathname'
 
@@ -96,7 +98,7 @@ def render_json_nil
     end
 
     def render_json_render_to_string
-      render :text => render_to_string(:json => '[]')
+      render :plain => render_to_string(:json => '[]')
     end
 
     def render_json_hello_world
@@ -133,28 +135,28 @@ def render_json_without_options
     end
 
     def render_json_with_serializer
-      @current_user = Struct.new(:as_json).new(:current_user => true)
+      @current_user = Struct.new(:as_json).new({:current_user => true})
       render :json => JsonSerializable.new
     end
 
     def render_json_with_serializer_and_implicit_root
-      @current_user = Struct.new(:as_json).new(:current_user => true)
+      @current_user = Struct.new(:as_json).new({:current_user => true})
       render :json => [JsonSerializable.new]
     end
 
     def render_json_with_serializer_and_options
-      @current_user = Struct.new(:as_json).new(:current_user => true)
+      @current_user = Struct.new(:as_json).new({:current_user => true})
       render :json => JsonSerializable.new, :options => true
     end
 
     def render_json_with_serializer_and_scope_option
-      @current_user = Struct.new(:as_json).new(:current_user => true)
-      scope = Struct.new(:as_json).new(:current_user => false)
+      @current_user = Struct.new(:as_json).new({:current_user => true})
+      scope = Struct.new(:as_json).new({:current_user => false})
       render :json => JsonSerializable.new, :scope => scope
     end
 
     def render_json_with_serializer_api_but_without_serializer
-      @current_user = Struct.new(:as_json).new(:current_user => true)
+      @current_user = Struct.new(:as_json).new({:current_user => true})
       render :json => JsonSerializable.new(true)
     end
 
@@ -215,7 +217,7 @@ def setup
   def test_render_json_nil
     get :render_json_nil
     assert_equal 'null', @response.body
-    assert_equal 'application/json', @response.content_type
+    assert_match %r{application/json}, @response.content_type
   end
 
   def test_render_json_render_to_string
@@ -231,7 +233,7 @@ def test_render_json_nil_with_custom_serializer
   def test_render_json
     get :render_json_hello_world
     assert_equal '{"hello":"world"}', @response.body
-    assert_equal 'application/json', @response.content_type
+    assert_match %r{application/json}, @response.content_type
   end
 
   def test_render_json_with_status
@@ -244,29 +246,29 @@ def test_render_json_with_callback
     get :render_json_hello_world_with_callback
     if Rails::VERSION::MAJOR == 3
       assert_equal 'alert({"hello":"world"})', @response.body
-      assert_match %r(application/json), @response.content_type.to_s
+      assert_match %r{application/json}, @response.content_type.to_s
     else
       assert_equal '/**/alert({"hello":"world"})', @response.body
-      assert_match %r(text/javascript), @response.content_type.to_s
+      assert_match %r{text/javascript}, @response.content_type.to_s
     end
   end
 
   def test_render_json_with_custom_content_type
     get :render_json_with_custom_content_type
     assert_equal '{"hello":"world"}', @response.body
-    assert_equal 'text/javascript', @response.content_type
+    assert_match %r{text/javascript}, @response.content_type
   end
 
   def test_render_symbol_json
     get :render_symbol_json
     assert_equal '{"hello":"world"}', @response.body
-    assert_equal 'application/json', @response.content_type
+    assert_match %r{application/json}, @response.content_type
   end
 
   def test_render_json_forwards_extra_options
     get :render_json_with_extra_options
     assert_equal '{"a":"b"}', @response.body
-    assert_equal 'application/json', @response.content_type
+    assert_match %r{application/json}, @response.content_type
   end
 
   def test_render_json_calls_to_json_from_object
@@ -280,33 +282,11 @@ def test_render_json_with_serializer
     assert_match '"object":{"serializable_object":true}', @response.body
   end
 
-  def test_render_json_with_serializer_checking_defaults
-    get :render_json_with_serializer, :check_defaults => true
-    assert_match '"scope":{"current_user":true}', @response.body
-    assert_match '"object":{"serializable_object":true}', @response.body
-    assert_match '"check_defaults":true', @response.body
-  end
-
-  def test_render_json_with_serializer_checking_default_serailizer
-    get :render_json_with_serializer, :check_default_serializer => true
-    assert_match '{"rails":"rocks"}', @response.body
-  end
-
-  def test_render_json_with_serializer_checking_default_scope
-    get :render_json_with_serializer, :check_default_scope => true
-    assert_match '"scope":"current_admin"', @response.body
-  end
-
   def test_render_json_with_serializer_and_implicit_root
     get :render_json_with_serializer_and_implicit_root
     assert_match '"test":[{"serializable_object":true}]', @response.body
   end
 
-  def test_render_json_with_serializer_and_implicit_root_checking_default_each_serailizer
-    get :render_json_with_serializer_and_implicit_root, :check_default_each_serializer => true
-    assert_match '"test":[{"rails":"rocks"}]', @response.body
-  end
-
   def test_render_json_with_serializer_and_options
     get :render_json_with_serializer_and_options
     assert_match '"scope":{"current_user":true}', @response.body
@@ -319,11 +299,6 @@ def test_render_json_with_serializer_and_scope_option
     assert_match '"scope":{"current_user":false}', @response.body
   end
 
-  def test_render_json_with_serializer_and_scope_option_checking_default_scope
-    get :render_json_with_serializer_and_scope_option, :check_default_scope => true
-    assert_match '"scope":{"current_user":false}', @response.body
-  end
-
   def test_render_json_with_serializer_api_but_without_serializer
     get :render_json_with_serializer_api_but_without_serializer
     assert_match '{"serializable_object":true}', @response.body
@@ -334,11 +309,6 @@ def test_render_json_with_custom_serializer
     assert_match '{"hello":true}', @response.body
   end
 
-  def test_render_json_with_custom_serializer_checking_default_serailizer
-    get :render_json_with_custom_serializer, :check_default_serializer => true
-    assert_match '{"hello":true}', @response.body
-  end
-
   def test_render_json_array_with_custom_serializer
     get :render_json_array_with_custom_serializer
     assert_match '{"test":[{"hello":true}]}', @response.body
@@ -350,11 +320,6 @@ def test_render_json_array_with_wrong_option
     end
   end
 
-  def test_render_json_array_with_custom_serializer_checking_default_each_serailizer
-    get :render_json_array_with_custom_serializer, :check_default_each_serializer => true
-    assert_match '{"test":[{"hello":true}]}', @response.body
-  end
-
   def test_render_json_with_links
     get :render_json_with_links
     assert_match '{"link":"http://www.nextangle.com/hypermedia"}', @response.body
@@ -365,21 +330,11 @@ def test_render_json_array_with_no_root
     assert_equal '[]', @response.body
   end
 
-  def test_render_json_array_with_no_root_checking_default_root
-    get :render_json_array_with_no_root, :check_default_root => true
-    assert_equal '[]', @response.body
-  end
-
   def test_render_json_empty_array
     get :render_json_empty_array
     assert_equal '{"test":[]}', @response.body
   end
 
-  def test_render_json_empty_array_checking_default_root
-    get :render_json_empty_array, :check_default_root => true
-    assert_equal '{"awesome":[]}', @response.body
-  end
-
   def test_render_json_empty_array_with_array_serializer_root_false
     ActiveModel::ArraySerializer.root = false
     get :render_json_empty_array
diff --git a/test/serializer_support_test.rb b/test/serializer_support_test.rb
index f90fa45f2..d7ebcdc0c 100644
--- a/test/serializer_support_test.rb
+++ b/test/serializer_support_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "test_helper"
 
 class RandomModel
@@ -27,7 +29,7 @@ class Criteria
 
 class SerializerSupportTest < ActiveSupport::TestCase
   test "it returns nil if no serializer exists" do
-    assert_equal nil, RandomModel.new.active_model_serializer
+    assert_nil RandomModel.new.active_model_serializer
   end
 
   test "it returns a deducted serializer if it exists exists" do
diff --git a/test/serializer_test.rb b/test/serializer_test.rb
index d8695942b..55619427d 100644
--- a/test/serializer_test.rb
+++ b/test/serializer_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "test_helper"
 require "test_fakes"
 
@@ -295,7 +297,7 @@ def test_nil_root_object
       root false
     end
 
-    assert_equal(nil, serializer.new(blog, :scope => user).as_json)
+    assert_nil(serializer.new(blog, :scope => user).as_json)
   end
 
   def test_custom_root_with_nil_root_object
@@ -442,10 +444,8 @@ def comments
     comments = [Comment.new(:title => "Comment1", :id => 1), Comment.new(:title => "Comment2", :id => 2)]
     post.comments = comments
 
-    post.class_eval do
-      define_method :comment_ids, lambda {
-        self.comments.map { |c| c.read_attribute_for_serialization(:id) }
-      }
+    post.define_singleton_method(:comment_ids) do
+      self.comments.map { |c| c.read_attribute_for_serialization(:id) }
     end
     json = post_serializer.new(post).as_json
     assert_equal({
diff --git a/test/support/rails5_shims.rb b/test/support/rails5_shims.rb
index 03a036da6..55de76e96 100644
--- a/test/support/rails5_shims.rb
+++ b/test/support/rails5_shims.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Rails5Shims
   module ControllerTests
     # https://github.com/rails/rails/blob/b217354/actionpack/lib/action_controller/test_case.rb
diff --git a/test/test_fakes.rb b/test/test_fakes.rb
index 366790e02..077dffdc2 100644
--- a/test/test_fakes.rb
+++ b/test/test_fakes.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class BasicActiveModel
   include ActiveModel::Serializers::JSON
   def initialize(hash = {})
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 061f64fae..3790ab431 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rubygems"
 require "bundler/setup"