diff --git a/lib/active_model_serializers.rb b/lib/active_model_serializers.rb
index b55dae35..18cdd9f7 100644
--- a/lib/active_model_serializers.rb
+++ b/lib/active_model_serializers.rb
@@ -38,6 +38,14 @@ module ActiveModelSerializers
     @default_include_directive ||= JSONAPI::IncludeDirective.new(config.default_includes, allow_wildcard: true)
   end
 
+  def self.silence_warnings
+    original_verbose = $VERBOSE
+    $VERBOSE = nil
+    yield
+  ensure
+    $VERBOSE = original_verbose
+  end
+
   require 'active_model/serializer/version'
   require 'active_model/serializer'
   require 'active_model/serializable_resource'
diff --git a/lib/active_model_serializers/model.rb b/lib/active_model_serializers/model.rb
index 2abe3334..0bdeda21 100644
--- a/lib/active_model_serializers/model.rb
+++ b/lib/active_model_serializers/model.rb
@@ -3,42 +3,58 @@
 # serializable non-activerecord objects.
 module ActiveModelSerializers
   class Model
-    include ActiveModel::Model
     include ActiveModel::Serializers::JSON
+    include ActiveModel::Model
+
+    class_attribute :attribute_names
+    # Initialize +attribute_names+ for all subclasses.  The array is usually
+    # mutated in the +attributes+ method, but can be set directly, as well.
+    self.attribute_names = []
 
     def self.attributes(*names)
-      attr_accessor(*names)
+      self.attribute_names |= names.map(&:to_sym)
+      # Silence redefinition of methods warnings
+      ActiveModelSerializers.silence_warnings do
+        attr_accessor(*names)
+      end
     end
 
-    attr_reader :attributes, :errors
+    attr_reader :errors
+    # NOTE that +updated_at+ isn't included in +attribute_names+,
+    # which means it won't show up in +attributes+ unless a subclass has
+    # either <tt>attributes :updated_at</tt> which will redefine the methods
+    # or <tt>attribute_names << :updated_at</tt>.
+    attr_writer :updated_at
+    # NOTE that +id+ will always be in +attributes+.
+    attributes :id
 
     def initialize(attributes = {})
-      @attributes = attributes && attributes.symbolize_keys
       @errors = ActiveModel::Errors.new(self)
       super
     end
 
-    # Defaults to the downcased model name.
-    def id
-      attributes.fetch(:id) { self.class.name.downcase }
+    # The the fields in +attribute_names+ determines the returned hash.
+    # +attributes+ are returned frozen to prevent any expectations that mutation affects
+    # the actual values in the model.
+    def attributes
+      attribute_names.each_with_object({}) do |attribute_name, result|
+        result[attribute_name] = public_send(attribute_name).freeze
+      end.with_indifferent_access.freeze
     end
 
-    # Defaults to the downcased model name and updated_at
+    # To customize model behavior, this method must be redefined. However,
+    # there are other ways of setting the +cache_key+ a serializer uses.
     def cache_key
-      attributes.fetch(:cache_key) { "#{self.class.name.downcase}/#{id}-#{updated_at.strftime('%Y%m%d%H%M%S%9N')}" }
+      ActiveSupport::Cache.expand_cache_key([
+        self.class.model_name.name.downcase,
+        "#{id}-#{updated_at.strftime('%Y%m%d%H%M%S%9N')}"
+      ].compact)
     end
 
-    # Defaults to the time the serializer file was modified.
+    # When no set, defaults to the time the file was modified.
+    # See NOTE by attr_writer :updated_at
     def updated_at
-      attributes.fetch(:updated_at) { File.mtime(__FILE__) }
-    end
-
-    def read_attribute_for_serialization(key)
-      if key == :id || key == 'id'
-        attributes.fetch(key) { id }
-      else
-        attributes[key]
-      end
+      defined?(@updated_at) ? @updated_at : File.mtime(__FILE__)
     end
 
     # The following methods are needed to be minimally implemented for ActiveModel::Errors
diff --git a/test/action_controller/adapter_selector_test.rb b/test/action_controller/adapter_selector_test.rb
index 2746943f..6f22aae2 100644
--- a/test/action_controller/adapter_selector_test.rb
+++ b/test/action_controller/adapter_selector_test.rb
@@ -15,7 +15,7 @@ module ActionController
         end
 
         def render_skipping_adapter
-          @profile = Profile.new(name: 'Name 1', description: 'Description 1', comments: 'Comments 1')
+          @profile = Profile.new(id: 'render_skipping_adapter_id', name: 'Name 1', description: 'Description 1', comments: 'Comments 1')
           render json: @profile, adapter: false
         end
       end
@@ -46,7 +46,7 @@ module ActionController
 
       def test_render_skipping_adapter
         get :render_skipping_adapter
-        assert_equal '{"name":"Name 1","description":"Description 1","comments":"Comments 1"}', response.body
+        assert_equal '{"id":"render_skipping_adapter_id","name":"Name 1","description":"Description 1"}', response.body
       end
     end
   end
diff --git a/test/action_controller/json_api/fields_test.rb b/test/action_controller/json_api/fields_test.rb
index 4bf08c7e..af87ad39 100644
--- a/test/action_controller/json_api/fields_test.rb
+++ b/test/action_controller/json_api/fields_test.rb
@@ -5,7 +5,16 @@ module ActionController
     class JsonApi
       class FieldsTest < ActionController::TestCase
         class FieldsTestController < ActionController::Base
-          class PostSerializer < ActiveModel::Serializer
+          class AuthorWithName < Author
+            attributes :first_name, :last_name
+          end
+          class AuthorWithNameSerializer < AuthorSerializer
+            type 'authors'
+          end
+          class PostWithPublishAt < Post
+            attributes :publish_at
+          end
+          class PostWithPublishAtSerializer < ActiveModel::Serializer
             type 'posts'
             attributes :title, :body, :publish_at
             belongs_to :author
@@ -14,19 +23,19 @@ module ActionController
 
           def setup_post
             ActionController::Base.cache_store.clear
-            @author = Author.new(id: 1, first_name: 'Bob', last_name: 'Jones')
+            @author = AuthorWithName.new(id: 1, first_name: 'Bob', last_name: 'Jones')
             @comment1 = Comment.new(id: 7, body: 'cool', author: @author)
             @comment2 = Comment.new(id: 12, body: 'awesome', author: @author)
-            @post = Post.new(id: 1337, title: 'Title 1', body: 'Body 1',
-                             author: @author, comments: [@comment1, @comment2],
-                             publish_at: '2020-03-16T03:55:25.291Z')
+            @post = PostWithPublishAt.new(id: 1337, title: 'Title 1', body: 'Body 1',
+                                          author: @author, comments: [@comment1, @comment2],
+                                          publish_at: '2020-03-16T03:55:25.291Z')
             @comment1.post = @post
             @comment2.post = @post
           end
 
           def render_fields_works_on_relationships
             setup_post
-            render json: @post, serializer: PostSerializer, adapter: :json_api, fields: { posts: [:author] }
+            render json: @post, serializer: PostWithPublishAtSerializer, adapter: :json_api, fields: { posts: [:author] }
           end
         end
 
diff --git a/test/action_controller/json_api/transform_test.rb b/test/action_controller/json_api/transform_test.rb
index 492b606b..69212f32 100644
--- a/test/action_controller/json_api/transform_test.rb
+++ b/test/action_controller/json_api/transform_test.rb
@@ -5,9 +5,17 @@ module ActionController
     class JsonApi
       class KeyTransformTest < ActionController::TestCase
         class KeyTransformTestController < ActionController::Base
-          class Post < ::Model; end
-          class Author < ::Model; end
-          class TopComment < ::Model; end
+          class Post < ::Model
+            attributes :title, :body, :publish_at
+            associations :author, :top_comments
+          end
+          class Author < ::Model
+            attributes :first_name, :last_name
+          end
+          class TopComment < ::Model
+            attributes :body
+            associations :author, :post
+          end
           class PostSerializer < ActiveModel::Serializer
             type 'posts'
             attributes :title, :body, :publish_at
diff --git a/test/action_controller/namespace_lookup_test.rb b/test/action_controller/namespace_lookup_test.rb
index 3203fd0b..f40cca11 100644
--- a/test/action_controller/namespace_lookup_test.rb
+++ b/test/action_controller/namespace_lookup_test.rb
@@ -3,10 +3,16 @@ require 'test_helper'
 module ActionController
   module Serialization
     class NamespaceLookupTest < ActionController::TestCase
-      class Book < ::Model; end
-      class Page < ::Model; end
-      class Chapter < ::Model; end
-      class Writer < ::Model; end
+      class Book < ::Model
+        attributes :title, :body
+        associations :writer, :chapters
+      end
+      class Chapter < ::Model
+        attributes :title
+      end
+      class Writer < ::Model
+        attributes :name
+      end
 
       module Api
         module V2
@@ -93,7 +99,7 @@ module ActionController
             end
 
             def invalid_namespace
-              book = Book.new(title: 'New Post', body: 'Body')
+              book = Book.new(id: 'invalid_namespace_book_id', title: 'New Post', body: 'Body')
 
               render json: book, namespace: :api_v2
             end
@@ -205,7 +211,7 @@ module ActionController
 
         assert_serializer ActiveModel::Serializer::Null
 
-        expected = { 'title' => 'New Post', 'body' => 'Body' }
+        expected = { 'id' => 'invalid_namespace_book_id', 'title' => 'New Post', 'body' => 'Body' }
         actual = JSON.parse(@response.body)
 
         assert_equal expected, actual
diff --git a/test/action_controller/serialization_test.rb b/test/action_controller/serialization_test.rb
index b5900e1d..e650cbfd 100644
--- a/test/action_controller/serialization_test.rb
+++ b/test/action_controller/serialization_test.rb
@@ -135,7 +135,7 @@ module ActionController
           like = Like.new(id: 1, likeable: comment, time: 3.days.ago)
 
           generate_cached_serializer(like)
-          like.likable = comment2
+          like.likeable = comment2
           like.time = Time.zone.now.to_s
 
           render json: like
diff --git a/test/adapter/json_api/fields_test.rb b/test/adapter/json_api/fields_test.rb
index 8aea4a1d..85228318 100644
--- a/test/adapter/json_api/fields_test.rb
+++ b/test/adapter/json_api/fields_test.rb
@@ -4,9 +4,17 @@ module ActiveModelSerializers
   module Adapter
     class JsonApi
       class FieldsTest < ActiveSupport::TestCase
-        class Post < ::Model; end
-        class Author < ::Model; end
-        class Comment < ::Model; end
+        class Post < ::Model
+          attributes :title, :body
+          associations :author, :comments
+        end
+        class Author < ::Model
+          attributes :name, :birthday
+        end
+        class Comment < ::Model
+          attributes :body
+          associations :author, :post
+        end
 
         class PostSerializer < ActiveModel::Serializer
           type 'posts'
diff --git a/test/adapter/json_api/include_data_if_sideloaded_test.rb b/test/adapter/json_api/include_data_if_sideloaded_test.rb
index 1c97191d..728eae13 100644
--- a/test/adapter/json_api/include_data_if_sideloaded_test.rb
+++ b/test/adapter/json_api/include_data_if_sideloaded_test.rb
@@ -5,7 +5,9 @@ module ActiveModel
     module Adapter
       class JsonApi
         class IncludeParamTest < ActiveSupport::TestCase
-          IncludeParamAuthor = Class.new(::Model)
+          IncludeParamAuthor = Class.new(::Model) do
+            associations :tags, :posts
+          end
 
           class CustomCommentLoader
             def all
diff --git a/test/adapter/json_api/linked_test.rb b/test/adapter/json_api/linked_test.rb
index 949bcf60..0d9c69b6 100644
--- a/test/adapter/json_api/linked_test.rb
+++ b/test/adapter/json_api/linked_test.rb
@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class NestedPost < ::Model; end
+class NestedPost < ::Model; associations :nested_posts end
 class NestedPostSerializer < ActiveModel::Serializer
   has_many :nested_posts
 end
@@ -301,8 +301,8 @@ module ActiveModelSerializers
       end
 
       class NoDuplicatesTest < ActiveSupport::TestCase
-        class Post < ::Model; end
-        class Author < ::Model; end
+        class Post < ::Model; associations :author end
+        class Author < ::Model; associations :posts, :roles, :bio end
 
         class PostSerializer < ActiveModel::Serializer
           type 'posts'
diff --git a/test/adapter/json_api/links_test.rb b/test/adapter/json_api/links_test.rb
index 3534b03c..ffbfa303 100644
--- a/test/adapter/json_api/links_test.rb
+++ b/test/adapter/json_api/links_test.rb
@@ -4,7 +4,7 @@ module ActiveModelSerializers
   module Adapter
     class JsonApi
       class LinksTest < ActiveSupport::TestCase
-        class LinkAuthor < ::Model; end
+        class LinkAuthor < ::Model; associations :posts end
         class LinkAuthorSerializer < ActiveModel::Serializer
           link :self do
             href "http://example.com/link_author/#{object.id}"
diff --git a/test/adapter/json_api/transform_test.rb b/test/adapter/json_api/transform_test.rb
index 47488d29..887ec835 100644
--- a/test/adapter/json_api/transform_test.rb
+++ b/test/adapter/json_api/transform_test.rb
@@ -4,9 +4,17 @@ module ActiveModelSerializers
   module Adapter
     class JsonApi
       class KeyCaseTest < ActiveSupport::TestCase
-        class Post < ::Model; end
-        class Author < ::Model; end
-        class Comment < ::Model; end
+        class Post < ::Model
+          attributes :title, :body, :publish_at
+          associations :author, :comments
+        end
+        class Author < ::Model
+          attributes :first_name, :last_name
+        end
+        class Comment < ::Model
+          attributes :body
+          associations :author, :post
+        end
 
         class PostSerializer < ActiveModel::Serializer
           type 'posts'
diff --git a/test/cache_test.rb b/test/cache_test.rb
index 44527378..b2cb27eb 100644
--- a/test/cache_test.rb
+++ b/test/cache_test.rb
@@ -34,6 +34,7 @@ module ActiveModelSerializers
     end
 
     class Article < ::Model
+      attributes :title
       # To confirm error is raised when cache_key is not set and cache_key option not passed to cache
       undef_method :cache_key
     end
@@ -48,6 +49,16 @@ module ActiveModelSerializers
       attribute :special_attribute
     end
 
+    class Comment < ::Model
+      attributes :body
+      associations :post, :author
+
+      # Uses a custom non-time-based cache key
+      def cache_key
+        "comment/#{id}"
+      end
+    end
+
     setup do
       cache_store.clear
       @comment        = Comment.new(id: 1, body: 'ZOMG A COMMENT')
@@ -271,7 +282,7 @@ module ActiveModelSerializers
         ended_at: nil,
         updated_at: alert.updated_at,
         created_at: alert.created_at
-      }
+      }.with_indifferent_access
       expected_cached_jsonapi_attributes = {
         id: '1',
         type: 'alerts',
@@ -283,15 +294,15 @@ module ActiveModelSerializers
           updated_at: alert.updated_at,
           created_at: alert.created_at
         }
-      }
+      }.with_indifferent_access
 
       # Assert attributes are serialized correctly
       serializable_alert = serializable(alert, serializer: AlertSerializer, adapter: :attributes)
-      attributes_serialization = serializable_alert.as_json
+      attributes_serialization = serializable_alert.as_json.with_indifferent_access
       assert_equal expected_fetch_attributes, alert.attributes
       assert_equal alert.attributes, attributes_serialization
       attributes_cache_key = serializable_alert.adapter.serializer.cache_key(serializable_alert.adapter)
-      assert_equal attributes_serialization, cache_store.fetch(attributes_cache_key)
+      assert_equal attributes_serialization, cache_store.fetch(attributes_cache_key).with_indifferent_access
 
       serializable_alert = serializable(alert, serializer: AlertSerializer, adapter: :json_api)
       jsonapi_cache_key = serializable_alert.adapter.serializer.cache_key(serializable_alert.adapter)
@@ -303,7 +314,7 @@ module ActiveModelSerializers
       serializable_alert = serializable(alert, serializer: UncachedAlertSerializer, adapter: :json_api)
       assert_equal serializable_alert.as_json, jsonapi_serialization
 
-      cached_serialization = cache_store.fetch(jsonapi_cache_key)
+      cached_serialization = cache_store.fetch(jsonapi_cache_key).with_indifferent_access
       assert_equal expected_cached_jsonapi_attributes, cached_serialization
     ensure
       Object.send(:remove_const, :Alert)
@@ -329,11 +340,15 @@ module ActiveModelSerializers
       actual = ActiveModel::Serializer.object_cache_keys(serializable.adapter.serializer, serializable.adapter, include_directive)
 
       assert_equal 3, actual.size
-      assert actual.any? { |key| key == "comment/1/#{serializable.adapter.cache_key}" }
-      assert actual.any? { |key| key =~ %r{post/post-\d+} }
-      assert actual.any? { |key| key =~ %r{author/author-\d+} }
+      expected_key = "comment/1/#{serializable.adapter.cache_key}"
+      assert actual.any? { |key| key == expected_key }, "actual '#{actual}' should include #{expected_key}"
+      expected_key = %r{post/post-\d+}
+      assert actual.any? { |key| key =~ expected_key }, "actual '#{actual}' should match '#{expected_key}'"
+      expected_key = %r{author/author-\d+}
+      assert actual.any? { |key| key =~ expected_key }, "actual '#{actual}' should match '#{expected_key}'"
     end
 
+    # rubocop:disable Metrics/AbcSize
     def test_fetch_attributes_from_cache
       serializers = ActiveModel::Serializer::CollectionSerializer.new([@comment, @comment])
 
@@ -344,10 +359,10 @@ module ActiveModelSerializers
         adapter_options = {}
         adapter_instance = ActiveModelSerializers::Adapter::Attributes.new(serializers, adapter_options)
         serializers.serializable_hash(adapter_options, options, adapter_instance)
-        cached_attributes = adapter_options.fetch(:cached_attributes)
+        cached_attributes = adapter_options.fetch(:cached_attributes).with_indifferent_access
 
         include_directive = ActiveModelSerializers.default_include_directive
-        manual_cached_attributes = ActiveModel::Serializer.cache_read_multi(serializers, adapter_instance, include_directive)
+        manual_cached_attributes = ActiveModel::Serializer.cache_read_multi(serializers, adapter_instance, include_directive).with_indifferent_access
         assert_equal manual_cached_attributes, cached_attributes
 
         assert_equal cached_attributes["#{@comment.cache_key}/#{adapter_instance.cache_key}"], Comment.new(id: 1, body: 'ZOMG A COMMENT').attributes
@@ -358,6 +373,7 @@ module ActiveModelSerializers
         assert_equal cached_attributes["#{writer_cache_key}/#{adapter_instance.cache_key}"], Author.new(id: 'author', name: 'Joao M. D. Moura').attributes
       end
     end
+    # rubocop:enable Metrics/AbcSize
 
     def test_cache_read_multi_with_fragment_cache_enabled
       post_serializer = Class.new(ActiveModel::Serializer) do
@@ -516,7 +532,7 @@ module ActiveModelSerializers
       role_hash = role_serializer.fetch_attributes_fragment(adapter_instance)
       assert_equal(role_hash, expected_result)
 
-      role.attributes[:id] = 'this has been updated'
+      role.id = 'this has been updated'
       role.name = 'this was cached'
 
       role_hash = role_serializer.fetch_attributes_fragment(adapter_instance)
diff --git a/test/collection_serializer_test.rb b/test/collection_serializer_test.rb
index 7b2a33d2..cdbebb15 100644
--- a/test/collection_serializer_test.rb
+++ b/test/collection_serializer_test.rb
@@ -3,14 +3,27 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class CollectionSerializerTest < ActiveSupport::TestCase
+      class SingularModel < ::Model; end
+      class SingularModelSerializer < ActiveModel::Serializer
+      end
+      class HasManyModel < ::Model
+        associations :singular_models
+      end
+      class HasManyModelSerializer < ActiveModel::Serializer
+        has_many :singular_models
+
+        def custom_options
+          instance_options
+        end
+      end
       class MessagesSerializer < ActiveModel::Serializer
         type 'messages'
       end
 
       def setup
-        @comment = Comment.new
-        @post = Post.new
-        @resource = build_named_collection @comment, @post
+        @singular_model = SingularModel.new
+        @has_many_model = HasManyModel.new
+        @resource = build_named_collection @singular_model, @has_many_model
         @serializer = collection_serializer.new(@resource, some: :options)
       end
 
@@ -34,29 +47,29 @@ module ActiveModel
       def test_each_object_should_be_serialized_with_appropriate_serializer
         serializers =  @serializer.to_a
 
-        assert_kind_of CommentSerializer, serializers.first
-        assert_kind_of Comment, serializers.first.object
+        assert_kind_of SingularModelSerializer, serializers.first
+        assert_kind_of SingularModel, serializers.first.object
 
-        assert_kind_of PostSerializer, serializers.last
-        assert_kind_of Post, serializers.last.object
+        assert_kind_of HasManyModelSerializer, serializers.last
+        assert_kind_of HasManyModel, serializers.last.object
 
         assert_equal :options, serializers.last.custom_options[:some]
       end
 
       def test_serializer_option_not_passed_to_each_serializer
-        serializers = collection_serializer.new([@post], serializer: PostSerializer).to_a
+        serializers = collection_serializer.new([@has_many_model], serializer: HasManyModelSerializer).to_a
 
         refute serializers.first.custom_options.key?(:serializer)
       end
 
       def test_root_default
-        @serializer = collection_serializer.new([@comment, @post])
+        @serializer = collection_serializer.new([@singular_model, @has_many_model])
         assert_nil @serializer.root
       end
 
       def test_root
         expected =  'custom_root'
-        @serializer = collection_serializer.new([@comment, @post], root: expected)
+        @serializer = collection_serializer.new([@singular_model, @has_many_model], root: expected)
         assert_equal expected, @serializer.root
       end
 
diff --git a/test/fixtures/active_record.rb b/test/fixtures/active_record.rb
index 77ac030d..9dc3830d 100644
--- a/test/fixtures/active_record.rb
+++ b/test/fixtures/active_record.rb
@@ -47,16 +47,6 @@ module ARModels
     has_many :comments
     belongs_to :author
   end
-
-  class Comment < ActiveRecord::Base
-    belongs_to :post
-    belongs_to :author
-  end
-
-  class Author < ActiveRecord::Base
-    has_many :posts
-  end
-
   class PostSerializer < ActiveModel::Serializer
     attributes :id, :title, :body
 
@@ -64,15 +54,60 @@ module ARModels
     belongs_to :author
   end
 
+  class Comment < ActiveRecord::Base
+    belongs_to :post
+    belongs_to :author
+  end
   class CommentSerializer < ActiveModel::Serializer
     attributes :id, :contents
 
     belongs_to :author
   end
 
+  class Author < ActiveRecord::Base
+    has_many :posts
+  end
   class AuthorSerializer < ActiveModel::Serializer
     attributes :id, :name
 
     has_many :posts
   end
 end
+
+class Employee < ActiveRecord::Base
+  has_many :pictures, as: :imageable
+  has_many :object_tags, as: :taggable
+end
+
+class PolymorphicSimpleSerializer < ActiveModel::Serializer
+  attributes :id
+end
+
+class ObjectTag < ActiveRecord::Base
+  belongs_to :poly_tag
+  belongs_to :taggable, polymorphic: true
+end
+class PolymorphicObjectTagSerializer < ActiveModel::Serializer
+  attributes :id
+  has_many :taggable, serializer: PolymorphicSimpleSerializer, polymorphic: true
+end
+
+class PolyTag < ActiveRecord::Base
+  has_many :object_tags
+end
+class PolymorphicTagSerializer < ActiveModel::Serializer
+  attributes :id, :phrase
+  has_many :object_tags, serializer: PolymorphicObjectTagSerializer
+end
+
+class Picture < ActiveRecord::Base
+  belongs_to :imageable, polymorphic: true
+  has_many :object_tags, as: :taggable
+end
+class PolymorphicHasManySerializer < ActiveModel::Serializer
+  attributes :id, :name
+end
+class PolymorphicBelongsToSerializer < ActiveModel::Serializer
+  attributes :id, :title
+  has_one :imageable, serializer: PolymorphicHasManySerializer, polymorphic: true
+end
diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index b7b82dff..3c804ccc 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -1,25 +1,27 @@
-verbose = $VERBOSE
-$VERBOSE = nil
 class Model < ActiveModelSerializers::Model
   FILE_DIGEST = Digest::MD5.hexdigest(File.open(__FILE__).read)
 
-  ### Helper methods, not required to be serializable
+  # Defaults to the downcased model name.
+  def id
+    @id ||= self.class.model_name.name.downcase
+  end
 
-  # Convenience when not adding @attributes readers and writers
-  def method_missing(meth, *args)
-    if meth.to_s =~ /^(.*)=$/
-      attributes[Regexp.last_match(1).to_sym] = args[0]
-    elsif attributes.key?(meth)
-      attributes[meth]
-    else
-      super
+  # At this time, just for organization of intent
+  class_attribute :association_names
+  self.association_names = []
+
+  def self.associations(*names)
+    self.association_names |= names.map(&:to_sym)
+    # Silence redefinition of methods warnings
+    ActiveModelSerializers.silence_warnings do
+      attr_accessor(*names)
     end
   end
 
-  # required for ActiveModel::AttributeAssignment#_assign_attribute
-  # in Rails 5
-  def respond_to_missing?(method_name, _include_private = false)
-    attributes.key?(method_name.to_s.tr('=', '').to_sym) || super
+  def associations
+    association_names.each_with_object({}) do |association_name, result|
+      result[association_name] = public_send(association_name).freeze
+    end.with_indifferent_access.freeze
   end
 end
 
@@ -30,67 +32,59 @@ end
 #   model = ModelWithErrors.new
 #   model.validate!            # => ["cannot be nil"]
 #   model.errors.full_messages # => ["name cannot be nil"]
-class ModelWithErrors < ::ActiveModelSerializers::Model
+class ModelWithErrors < Model
   attributes :name
 end
 
 class Profile < Model
+  attributes :name, :description
+  associations :comments
 end
-
 class ProfileSerializer < ActiveModel::Serializer
   attributes :name, :description
-
-  # TODO: is this used anywhere?
-  def arguments_passed_in?
-    instance_options[:my_options] == :accessible
-  end
 end
-
 class ProfilePreviewSerializer < ActiveModel::Serializer
   attributes :name
 end
 
-class Post < Model; end
-class Like < Model; end
-class Author < Model; end
-class Bio < Model; end
-class Blog < Model; end
-class Role < Model; end
-class User < Model; end
-class Location < Model; end
-class Place < Model; end
-class Tag < Model; end
-class VirtualValue < Model; end
+class Author < Model
+  attributes :name
+  associations :posts, :bio, :roles, :comments
+end
+class AuthorSerializer < ActiveModel::Serializer
+  cache key: 'writer', skip_digest: true
+  attribute :id
+  attribute :name
+
+  has_many :posts
+  has_many :roles
+  has_one :bio
+end
+class AuthorPreviewSerializer < ActiveModel::Serializer
+  attributes :id
+  has_many :posts
+end
+
 class Comment < Model
-  # Uses a custom non-time-based cache key
-  def cache_key
-    "#{self.class.name.downcase}/#{id}"
-  end
+  attributes :body, :date
+  associations :post, :author, :likes
+end
+class CommentSerializer < ActiveModel::Serializer
+  cache expires_in: 1.day, skip_digest: true
+  attributes :id, :body
+  belongs_to :post
+  belongs_to :author
+end
+class CommentPreviewSerializer < ActiveModel::Serializer
+  attributes :id
+
+  belongs_to :post
 end
 
-class Employee < ActiveRecord::Base
-  has_many :pictures, as: :imageable
-  has_many :object_tags, as: :taggable
+class Post < Model
+  attributes :title, :body
+  associations :author, :comments, :blog, :tags, :related
 end
-
-class ObjectTag < ActiveRecord::Base
-  belongs_to :poly_tag
-  belongs_to :taggable, polymorphic: true
-end
-
-class Picture < ActiveRecord::Base
-  belongs_to :imageable, polymorphic: true
-  has_many :object_tags, as: :taggable
-end
-
-class PolyTag < ActiveRecord::Base
-  has_many :object_tags
-end
-
-module Spam
-  class UnrelatedLink < Model; end
-end
-
 class PostSerializer < ActiveModel::Serializer
   cache key: 'post', expires_in: 0.1, skip_digest: true
   attributes :id, :title, :body
@@ -102,58 +96,79 @@ class PostSerializer < ActiveModel::Serializer
   def blog
     Blog.new(id: 999, name: 'Custom blog')
   end
-
-  # TODO: is this used anywhere?
-  def custom_options
-    instance_options
-  end
 end
-
 class SpammyPostSerializer < ActiveModel::Serializer
   attributes :id
   has_many :related
 end
+class PostPreviewSerializer < ActiveModel::Serializer
+  attributes :title, :body, :id
 
-class CommentSerializer < ActiveModel::Serializer
-  cache expires_in: 1.day, skip_digest: true
-  attributes :id, :body
+  has_many :comments, serializer: ::CommentPreviewSerializer
+  belongs_to :author, serializer: ::AuthorPreviewSerializer
+end
+class PostWithTagsSerializer < ActiveModel::Serializer
+  attributes :id
+  has_many :tags
+end
+class PostWithCustomKeysSerializer < ActiveModel::Serializer
+  attributes :id
+  has_many :comments, key: :reviews
+  belongs_to :author, key: :writer
+  has_one :blog, key: :site
+end
+
+class Bio < Model
+  attributes :content, :rating
+  associations :author
+end
+class BioSerializer < ActiveModel::Serializer
+  cache except: [:content], skip_digest: true
+  attributes :id, :content, :rating
 
-  belongs_to :post
   belongs_to :author
-
-  def custom_options
-    instance_options
-  end
 end
 
-class AuthorSerializer < ActiveModel::Serializer
-  cache key: 'writer', skip_digest: true
+class Blog < Model
+  attributes :name, :type, :special_attribute
+  associations :writer, :articles
+end
+class BlogSerializer < ActiveModel::Serializer
+  cache key: 'blog'
+  attributes :id, :name
+
+  belongs_to :writer
+  has_many :articles
+end
+class AlternateBlogSerializer < ActiveModel::Serializer
   attribute :id
-  attribute :name
-
-  has_many :posts
-  has_many :roles
-  has_one :bio
+  attribute :name, key: :title
+end
+class CustomBlogSerializer < ActiveModel::Serializer
+  attribute :id
+  attribute :special_attribute
+  has_many :articles
 end
 
+class Role < Model
+  attributes :name, :description, :special_attribute
+  associations :author
+end
 class RoleSerializer < ActiveModel::Serializer
   cache only: [:name, :slug], skip_digest: true
   attributes :id, :name, :description
   attribute :friendly_id, key: :slug
+  belongs_to :author
 
   def friendly_id
     "#{object.name}-#{object.id}"
   end
-
-  belongs_to :author
 end
 
-class LikeSerializer < ActiveModel::Serializer
-  attributes :id, :time
-
-  belongs_to :likeable
+class Location < Model
+  attributes :lat, :lng
+  associations :place
 end
-
 class LocationSerializer < ActiveModel::Serializer
   cache only: [:address], skip_digest: true
   attributes :id, :lat, :lng
@@ -165,81 +180,40 @@ class LocationSerializer < ActiveModel::Serializer
   end
 end
 
+class Place < Model
+  attributes :name
+  associations :locations
+end
 class PlaceSerializer < ActiveModel::Serializer
   attributes :id, :name
-
   has_many :locations
 end
 
-class BioSerializer < ActiveModel::Serializer
-  cache except: [:content], skip_digest: true
-  attributes :id, :content, :rating
-
-  belongs_to :author
+class Like < Model
+  attributes :time
+  associations :likeable
+end
+class LikeSerializer < ActiveModel::Serializer
+  attributes :id, :time
+  belongs_to :likeable
 end
 
-class BlogSerializer < ActiveModel::Serializer
-  cache key: 'blog'
-  attributes :id, :name
-
-  belongs_to :writer
-  has_many :articles
-end
-
-class PaginatedSerializer < ActiveModel::Serializer::CollectionSerializer
-  def json_key
-    'paginated'
+module Spam
+  class UnrelatedLink < Model
+  end
+  class UnrelatedLinkSerializer < ActiveModel::Serializer
+    cache only: [:id]
+    attributes :id
   end
 end
 
-class AlternateBlogSerializer < ActiveModel::Serializer
-  attribute :id
-  attribute :name, key: :title
-end
-
-class CustomBlogSerializer < ActiveModel::Serializer
-  attribute :id
-  attribute :special_attribute
-
-  has_many :articles
-end
-
-class CommentPreviewSerializer < ActiveModel::Serializer
-  attributes :id
-
-  belongs_to :post
-end
-
-class AuthorPreviewSerializer < ActiveModel::Serializer
-  attributes :id
-
-  has_many :posts
-end
-
-class PostPreviewSerializer < ActiveModel::Serializer
-  attributes :title, :body, :id
-
-  has_many :comments, serializer: CommentPreviewSerializer
-  belongs_to :author, serializer: AuthorPreviewSerializer
-end
-
-class PostWithTagsSerializer < ActiveModel::Serializer
-  attributes :id
-
-  has_many :tags
-end
-
-class PostWithCustomKeysSerializer < ActiveModel::Serializer
-  attributes :id
-
-  has_many :comments, key: :reviews
-  belongs_to :author, key: :writer
-  has_one :blog, key: :site
+class Tag < Model
+  attributes :name
 end
 
+class VirtualValue < Model; end
 class VirtualValueSerializer < ActiveModel::Serializer
   attributes :id
-
   has_many :reviews, virtual_value: [{ type: 'reviews', id: '1' },
                                      { type: 'reviews', id: '2' }]
   has_one :maker, virtual_value: { type: 'makers', id: '1' }
@@ -251,36 +225,8 @@ class VirtualValueSerializer < ActiveModel::Serializer
   end
 end
 
-class PolymorphicHasManySerializer < ActiveModel::Serializer
-  attributes :id, :name
-end
-
-class PolymorphicBelongsToSerializer < ActiveModel::Serializer
-  attributes :id, :title
-
-  has_one :imageable, serializer: PolymorphicHasManySerializer, polymorphic: true
-end
-
-class PolymorphicSimpleSerializer < ActiveModel::Serializer
-  attributes :id
-end
-
-class PolymorphicObjectTagSerializer < ActiveModel::Serializer
-  attributes :id
-
-  has_many :taggable, serializer: PolymorphicSimpleSerializer, polymorphic: true
-end
-
-class PolymorphicTagSerializer < ActiveModel::Serializer
-  attributes :id, :phrase
-
-  has_many :object_tags, serializer: PolymorphicObjectTagSerializer
-end
-
-module Spam
-  class UnrelatedLinkSerializer < ActiveModel::Serializer
-    cache only: [:id]
-    attributes :id
+class PaginatedSerializer < ActiveModel::Serializer::CollectionSerializer
+  def json_key
+    'paginated'
   end
 end
-$VERBOSE = verbose
diff --git a/test/serializers/associations_test.rb b/test/serializers/associations_test.rb
index ee470385..6d6447c3 100644
--- a/test/serializers/associations_test.rb
+++ b/test/serializers/associations_test.rb
@@ -8,7 +8,7 @@ module ActiveModel
         @author.roles = []
         @blog = Blog.new(name: 'AMS Blog')
         @post = Post.new(title: 'New Post', body: 'Body')
-        @tag = Tag.new(name: '#hashtagged')
+        @tag = Tag.new(id: 'tagid', name: '#hashtagged')
         @comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
         @post.comments = [@comment]
         @post.tags = [@tag]
@@ -53,7 +53,7 @@ module ActiveModel
 
           assert_equal :tags, key
           assert_nil serializer
-          assert_equal [{ name: '#hashtagged' }].to_json, options[:virtual_value].to_json
+          assert_equal [{ id: 'tagid', name: '#hashtagged' }].to_json, options[:virtual_value].to_json
         end
       end
 
@@ -62,7 +62,13 @@ module ActiveModel
                       .associations
                       .detect { |assoc| assoc.key == :comments }
 
-        assert association.serializer.first.custom_options[:custom_options]
+        comment_serializer = association.serializer.first
+        class << comment_serializer
+          def custom_options
+            instance_options
+          end
+        end
+        assert comment_serializer.custom_options.fetch(:custom_options)
       end
 
       def test_belongs_to
@@ -159,7 +165,9 @@ module ActiveModel
 
       class NamespacedResourcesTest < ActiveSupport::TestCase
         class ResourceNamespace
-          class Post    < ::Model; end
+          class Post < ::Model
+            associations :comments, :author, :description
+          end
           class Comment < ::Model; end
           class Author  < ::Model; end
           class Description < ::Model; end
@@ -200,7 +208,9 @@ module ActiveModel
       end
 
       class NestedSerializersTest < ActiveSupport::TestCase
-        class Post    < ::Model; end
+        class Post < ::Model
+          associations :comments, :author, :description
+        end
         class Comment < ::Model; end
         class Author  < ::Model; end
         class Description < ::Model; end
@@ -240,7 +250,10 @@ module ActiveModel
 
         # rubocop:disable Metrics/AbcSize
         def test_conditional_associations
-          model = ::Model.new(true: true, false: false)
+          model = Class.new(::Model) do
+            attributes :true, :false
+            associations :association
+          end.new(true: true, false: false)
 
           scenarios = [
             { options: { if:     :true  }, included: true  },
diff --git a/test/serializers/attribute_test.rb b/test/serializers/attribute_test.rb
index c359d2f9..608898c3 100644
--- a/test/serializers/attribute_test.rb
+++ b/test/serializers/attribute_test.rb
@@ -81,7 +81,7 @@ module ActiveModel
         assert_equal('custom', hash[:blog][:id])
       end
 
-      class PostWithVirtualAttribute < ::Model; end
+      class PostWithVirtualAttribute < ::Model; attributes :first_name, :last_name end
       class PostWithVirtualAttributeSerializer < ActiveModel::Serializer
         attribute :name do
           "#{object.first_name} #{object.last_name}"
@@ -98,7 +98,9 @@ module ActiveModel
 
       # rubocop:disable Metrics/AbcSize
       def test_conditional_associations
-        model = ::Model.new(true: true, false: false)
+        model = Class.new(::Model) do
+          attributes :true, :false, :attribute
+        end.new(true: true, false: false)
 
         scenarios = [
           { options: { if:     :true  }, included: true  },
diff --git a/test/serializers/options_test.rb b/test/serializers/options_test.rb
index 092714ab..009388e3 100644
--- a/test/serializers/options_test.rb
+++ b/test/serializers/options_test.rb
@@ -3,18 +3,29 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class OptionsTest < ActiveSupport::TestCase
-      def setup
-        @profile = Profile.new(name: 'Name 1', description: 'Description 1')
+      class ModelWithOptions < ActiveModelSerializers::Model
+        attributes :name, :description
+      end
+      class ModelWithOptionsSerializer < ActiveModel::Serializer
+        attributes :name, :description
+
+        def arguments_passed_in?
+          instance_options[:my_options] == :accessible
+        end
+      end
+
+      setup do
+        @model_with_options = ModelWithOptions.new(name: 'Name 1', description: 'Description 1')
       end
 
       def test_options_are_accessible
-        @profile_serializer = ProfileSerializer.new(@profile, my_options: :accessible)
-        assert @profile_serializer.arguments_passed_in?
+        model_with_options_serializer = ModelWithOptionsSerializer.new(@model_with_options, my_options: :accessible)
+        assert model_with_options_serializer.arguments_passed_in?
       end
 
       def test_no_option_is_passed_in
-        @profile_serializer = ProfileSerializer.new(@profile)
-        refute @profile_serializer.arguments_passed_in?
+        model_with_options_serializer = ModelWithOptionsSerializer.new(@model_with_options)
+        refute model_with_options_serializer.arguments_passed_in?
       end
     end
   end
diff --git a/test/serializers/serializer_for_with_namespace_test.rb b/test/serializers/serializer_for_with_namespace_test.rb
index 5a8a9ed5..5c6e3e5e 100644
--- a/test/serializers/serializer_for_with_namespace_test.rb
+++ b/test/serializers/serializer_for_with_namespace_test.rb
@@ -3,9 +3,12 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class SerializerForWithNamespaceTest < ActiveSupport::TestCase
-      class Book < ::Model; end
-      class Page < ::Model; end
-      class Publisher < ::Model; end
+      class Book < ::Model
+        attributes :title, :author_name
+        associations :publisher, :pages
+      end
+      class Page < ::Model; attributes :number, :text end
+      class Publisher < ::Model; attributes :name end
 
       module Api
         module V3
@@ -18,8 +21,6 @@ module ActiveModel
 
           class PageSerializer < ActiveModel::Serializer
             attributes :number, :text
-
-            belongs_to :book
           end
 
           class PublisherSerializer < ActiveModel::Serializer