diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 50ac2bbd..570073c7 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -98,6 +98,22 @@ module ActiveModel
       end
     end
 
+    # @api private
+    def self.include_directive_from_options(options)
+      if options[:include_directive]
+        options[:include_directive]
+      elsif options[:include]
+        JSONAPI::IncludeDirective.new(options[:include], allow_wildcard: true)
+      else
+        ActiveModelSerializers.default_include_directive
+      end
+    end
+
+    # @api private
+    def self.serialization_adapter_instance
+      @serialization_adapter_instance ||= ActiveModelSerializers::Adapter::Attributes
+    end
+
     attr_accessor :object, :root, :scope
 
     # `scope_name` is set as :current_user by default in the controller.
@@ -123,9 +139,7 @@ module ActiveModel
     # associations, similar to how ActiveModel::Serializers::JSON is used
     # in ActiveRecord::Base.
     #
-    # TODO: Move to here the Attributes adapter logic for
-    # +serializable_hash_for_single_resource(options)+
-    # and include <tt>ActiveModel::Serializers::JSON</tt>.
+    # TODO: Include <tt>ActiveModel::Serializers::JSON</tt>.
     # So that the below is true:
     #   @param options [nil, Hash] The same valid options passed to `serializable_hash`
     #      (:only, :except, :methods, and :include).
@@ -149,11 +163,13 @@ module ActiveModel
     #     serializer.as_json(include: :posts)
     #     # Second level and higher order associations work as well:
     #     serializer.as_json(include: { posts: { include: { comments: { only: :body } }, only: :title } })
-    def serializable_hash(adapter_opts = nil)
-      adapter_opts ||= {}
-      adapter_opts = { include: '*', adapter: :attributes }.merge!(adapter_opts)
-      adapter = ActiveModelSerializers::Adapter.create(self, adapter_opts)
-      adapter.serializable_hash(adapter_opts)
+    def serializable_hash(adapter_options = nil, options = {}, adapter_instance = self.class.serialization_adapter_instance)
+      adapter_options ||= {}
+      options[:include_directive] ||= ActiveModel::Serializer.include_directive_from_options(adapter_options)
+      cached_attributes = adapter_options[:cached_attributes] ||= {}
+      resource = cached_attributes(options[:fields], cached_attributes, adapter_instance)
+      relationships = resource_relationships(adapter_options, options, adapter_instance)
+      resource.merge(relationships)
     end
     alias to_hash serializable_hash
     alias to_h serializable_hash
@@ -185,6 +201,35 @@ module ActiveModel
       end
     end
 
+    # @api private
+    def resource_relationships(adapter_options, options, adapter_instance)
+      relationships = {}
+      include_directive = options.fetch(:include_directive)
+      associations(include_directive).each do |association|
+        adapter_opts = adapter_options.merge(include_directive: include_directive[association.key])
+        relationships[association.key] ||= relationship_value_for(association, adapter_opts, adapter_instance)
+      end
+
+      relationships
+    end
+
+    # @api private
+    def relationship_value_for(association, adapter_options, adapter_instance)
+      return association.options[:virtual_value] if association.options[:virtual_value]
+      association_serializer = association.serializer
+      association_object = association_serializer && association_serializer.object
+      return unless association_object
+
+      relationship_value = association_serializer.serializable_hash(adapter_options, {}, adapter_instance)
+
+      if association.options[:polymorphic] && relationship_value
+        polymorphic_type = association_object.class.name.underscore
+        relationship_value = { type: polymorphic_type, polymorphic_type.to_sym => relationship_value }
+      end
+
+      relationship_value
+    end
+
     protected
 
     attr_accessor :instance_options
diff --git a/lib/active_model/serializer/caching.rb b/lib/active_model/serializer/caching.rb
index 0f1fffb2..5adf4ee3 100644
--- a/lib/active_model/serializer/caching.rb
+++ b/lib/active_model/serializer/caching.rb
@@ -318,7 +318,7 @@ module ActiveModel
 
         parts = []
         parts << object_cache_key
-        parts << adapter_instance.cached_name
+        parts << adapter_instance.cache_key
         parts << self.class._cache_digest unless self.class._skip_digest?
         @cache_key = parts.join('/')
       end
diff --git a/lib/active_model/serializer/collection_serializer.rb b/lib/active_model/serializer/collection_serializer.rb
index f026ebfe..bb84644c 100644
--- a/lib/active_model/serializer/collection_serializer.rb
+++ b/lib/active_model/serializer/collection_serializer.rb
@@ -11,22 +11,23 @@ module ActiveModel
         @object                  = resources
         @options                 = options
         @root                    = options[:root]
-        serializer_context_class = options.fetch(:serializer_context_class, ActiveModel::Serializer)
-        @serializers = resources.map do |resource|
-          serializer_class = options.fetch(:serializer) { serializer_context_class.serializer_for(resource) }
-
-          if serializer_class.nil? # rubocop:disable Style/GuardClause
-            fail NoSerializerError, "No serializer found for resource: #{resource.inspect}"
-          else
-            serializer_class.new(resource, options.except(:serializer))
-          end
-        end
+        @serializers             = serializers_from_resources
       end
 
       def success?
         true
       end
 
+      # @api private
+      def serializable_hash(adapter_options, options, adapter_instance)
+        include_directive = ActiveModel::Serializer.include_directive_from_options(adapter_options)
+        adapter_options[:cached_attributes] ||= ActiveModel::Serializer.cache_read_multi(self, adapter_instance, include_directive)
+        adapter_opts = adapter_options.merge(include_directive: include_directive)
+        serializers.map do |serializer|
+          serializer.serializable_hash(adapter_opts, options, adapter_instance)
+        end
+      end
+
       # TODO: unify naming of root, json_key, and _type.  Right now, a serializer's
       # json_key comes from the root option or the object's model name, by default.
       # But, if a dev defines a custom `json_key` method with an explicit value,
@@ -59,6 +60,25 @@ module ActiveModel
       protected
 
       attr_reader :serializers, :options
+
+      private
+
+      def serializers_from_resources
+        serializer_context_class = options.fetch(:serializer_context_class, ActiveModel::Serializer)
+        object.map do |resource|
+          serializer_from_resource(resource, serializer_context_class, options)
+        end
+      end
+
+      def serializer_from_resource(resource, serializer_context_class, options)
+        serializer_class = options.fetch(:serializer) { serializer_context_class.serializer_for(resource) }
+
+        if serializer_class.nil? # rubocop:disable Style/GuardClause
+          fail NoSerializerError, "No serializer found for resource: #{resource.inspect}"
+        else
+          serializer_class.new(resource, options.except(:serializer))
+        end
+      end
     end
   end
 end
diff --git a/lib/active_model_serializers/adapter.rb b/lib/active_model_serializers/adapter.rb
index 44c10db1..93e4c044 100644
--- a/lib/active_model_serializers/adapter.rb
+++ b/lib/active_model_serializers/adapter.rb
@@ -51,6 +51,10 @@ module ActiveModelSerializers
         self
       end
 
+      def registered_name(adapter_class)
+        ADAPTER_MAP.key adapter_class
+      end
+
       # @param  adapter [String, Symbol, Class] name to fetch adapter by
       # @return [ActiveModelSerializers::Adapter] subclass of Adapter
       # @raise  [UnknownAdapterError]
diff --git a/lib/active_model_serializers/adapter/attributes.rb b/lib/active_model_serializers/adapter/attributes.rb
index 56a155c3..f28295c0 100644
--- a/lib/active_model_serializers/adapter/attributes.rb
+++ b/lib/active_model_serializers/adapter/attributes.rb
@@ -1,65 +1,9 @@
 module ActiveModelSerializers
   module Adapter
     class Attributes < Base
-      def initialize(serializer, options = {})
-        super
-        @include_directive =
-          if options[:include_directive]
-            options[:include_directive]
-          elsif options[:include]
-            JSONAPI::IncludeDirective.new(options[:include], allow_wildcard: true)
-          else
-            ActiveModelSerializers.default_include_directive
-          end
-      end
-
       def serializable_hash(options = nil)
         options = serialization_options(options)
-
-        if serializer.respond_to?(:each)
-          serializable_hash_for_collection(options)
-        else
-          serializable_hash_for_single_resource(options)
-        end
-      end
-
-      private
-
-      def serializable_hash_for_collection(options)
-        instance_options[:cached_attributes] ||= ActiveModel::Serializer.cache_read_multi(serializer, self, @include_directive)
-        opts = instance_options.merge(include_directive: @include_directive)
-        serializer.map { |s| Attributes.new(s, opts).serializable_hash(options) }
-      end
-
-      def serializable_hash_for_single_resource(options)
-        cached_attributes = instance_options[:cached_attributes] || {}
-        resource = serializer.cached_attributes(options[:fields], cached_attributes, self)
-        relationships = resource_relationships(options)
-        resource.merge(relationships)
-      end
-
-      def resource_relationships(options)
-        relationships = {}
-        serializer.associations(@include_directive).each do |association|
-          relationships[association.key] ||= relationship_value_for(association, options)
-        end
-
-        relationships
-      end
-
-      def relationship_value_for(association, options)
-        return association.options[:virtual_value] if association.options[:virtual_value]
-        return unless association.serializer && association.serializer.object
-
-        opts = instance_options.merge(include_directive: @include_directive[association.key])
-        relationship_value = Attributes.new(association.serializer, opts).serializable_hash(options)
-
-        if association.options[:polymorphic] && relationship_value
-          polymorphic_type = association.serializer.object.class.name.underscore
-          relationship_value = { type: polymorphic_type, polymorphic_type.to_sym => relationship_value }
-        end
-
-        relationship_value
+        serializer.serializable_hash(instance_options, options, self)
       end
     end
   end
diff --git a/lib/active_model_serializers/adapter/base.rb b/lib/active_model_serializers/adapter/base.rb
index 8c5e73d3..7b60db70 100644
--- a/lib/active_model_serializers/adapter/base.rb
+++ b/lib/active_model_serializers/adapter/base.rb
@@ -8,6 +8,40 @@ module ActiveModelSerializers
         ActiveModelSerializers::Adapter.register(subclass)
       end
 
+      # Sets the default transform for the adapter.
+      #
+      # @return [Symbol] the default transform for the adapter
+      def self.default_key_transform
+        :unaltered
+      end
+
+      # Determines the transform to use in order of precedence:
+      #   adapter option, global config, adapter default.
+      #
+      # @param options [Object]
+      # @return [Symbol] the transform to use
+      def self.transform(options)
+        return options[:key_transform] if options && options[:key_transform]
+        ActiveModelSerializers.config.key_transform || default_key_transform
+      end
+
+      # Transforms the casing of the supplied value.
+      #
+      # @param value [Object] the value to be transformed
+      # @param options [Object] serializable resource options
+      # @return [Symbol] the default transform for the adapter
+      def self.transform_key_casing!(value, options)
+        KeyTransform.send(transform(options), value)
+      end
+
+      def self.cache_key
+        @cache_key ||= ActiveModelSerializers::Adapter.registered_name(self)
+      end
+
+      def self.fragment_cache(cached_hash, non_cached_hash)
+        non_cached_hash.merge cached_hash
+      end
+
       attr_reader :serializer, :instance_options
 
       def initialize(serializer, options = {})
@@ -15,10 +49,6 @@ module ActiveModelSerializers
         @instance_options = options
       end
 
-      def cached_name
-        @cached_name ||= self.class.name.demodulize.underscore
-      end
-
       # Subclasses that implement this method must first call
       #   options = serialization_options(options)
       def serializable_hash(_options = nil)
@@ -29,8 +59,12 @@ module ActiveModelSerializers
         serializable_hash(options)
       end
 
+      def cache_key
+        self.class.cache_key
+      end
+
       def fragment_cache(cached_hash, non_cached_hash)
-        non_cached_hash.merge cached_hash
+        self.class.fragment_cache(cached_hash, non_cached_hash)
       end
 
       private
@@ -44,34 +78,6 @@ module ActiveModelSerializers
       def root
         serializer.json_key.to_sym if serializer.json_key
       end
-
-      class << self
-        # Sets the default transform for the adapter.
-        #
-        # @return [Symbol] the default transform for the adapter
-        def default_key_transform
-          :unaltered
-        end
-
-        # Determines the transform to use in order of precedence:
-        #   adapter option, global config, adapter default.
-        #
-        # @param options [Object]
-        # @return [Symbol] the transform to use
-        def transform(options)
-          return options[:key_transform] if options && options[:key_transform]
-          ActiveModelSerializers.config.key_transform || default_key_transform
-        end
-
-        # Transforms the casing of the supplied value.
-        #
-        # @param value [Object] the value to be transformed
-        # @param options [Object] serializable resource options
-        # @return [Symbol] the default transform for the adapter
-        def transform_key_casing!(value, options)
-          KeyTransform.send(transform(options), value)
-        end
-      end
     end
   end
 end
diff --git a/lib/active_model_serializers/adapter/json_api.rb b/lib/active_model_serializers/adapter/json_api.rb
index eb7f39b7..5293078c 100644
--- a/lib/active_model_serializers/adapter/json_api.rb
+++ b/lib/active_model_serializers/adapter/json_api.rb
@@ -31,16 +31,27 @@ module ActiveModelSerializers
       autoload :Error
       autoload :Deserialization
 
+      def self.default_key_transform
+        :dash
+      end
+
+      def self.fragment_cache(cached_hash, non_cached_hash, root = true)
+        core_cached       = cached_hash.first
+        core_non_cached   = non_cached_hash.first
+        no_root_cache     = cached_hash.delete_if { |key, _value| key == core_cached[0] }
+        no_root_non_cache = non_cached_hash.delete_if { |key, _value| key == core_non_cached[0] }
+        cached_resource   = (core_cached[1]) ? core_cached[1].deep_merge(core_non_cached[1]) : core_non_cached[1]
+        hash = root ? { root => cached_resource } : cached_resource
+
+        hash.deep_merge no_root_non_cache.deep_merge no_root_cache
+      end
+
       def initialize(serializer, options = {})
         super
         @include_directive = JSONAPI::IncludeDirective.new(options[:include], allow_wildcard: true)
         @fieldset = options[:fieldset] || ActiveModel::Serializer::Fieldset.new(options.delete(:fields))
       end
 
-      def self.default_key_transform
-        :dash
-      end
-
       # {http://jsonapi.org/format/#crud Requests are transactional, i.e. success or failure}
       # {http://jsonapi.org/format/#document-top-level data and errors MUST NOT coexist in the same document.}
       def serializable_hash(*)
@@ -52,6 +63,11 @@ module ActiveModelSerializers
         self.class.transform_key_casing!(document, instance_options)
       end
 
+      def fragment_cache(cached_hash, non_cached_hash)
+        root = !instance_options.include?(:include)
+        self.class.fragment_cache(cached_hash, non_cached_hash, root)
+      end
+
       # {http://jsonapi.org/format/#document-top-level Primary data}
       # definition:
       #   ☐ toplevel_data (required)
@@ -174,18 +190,6 @@ module ActiveModelSerializers
         hash
       end
 
-      def fragment_cache(cached_hash, non_cached_hash)
-        root = false if instance_options.include?(:include)
-        core_cached       = cached_hash.first
-        core_non_cached   = non_cached_hash.first
-        no_root_cache     = cached_hash.delete_if { |key, _value| key == core_cached[0] }
-        no_root_non_cache = non_cached_hash.delete_if { |key, _value| key == core_non_cached[0] }
-        cached_resource   = (core_cached[1]) ? core_cached[1].deep_merge(core_non_cached[1]) : core_non_cached[1]
-        hash = root ? { root => cached_resource } : cached_resource
-
-        hash.deep_merge no_root_non_cache.deep_merge no_root_cache
-      end
-
       protected
 
       attr_reader :fieldset
diff --git a/test/action_controller/adapter_selector_test.rb b/test/action_controller/adapter_selector_test.rb
index aa3539f7..f392b4a4 100644
--- a/test/action_controller/adapter_selector_test.rb
+++ b/test/action_controller/adapter_selector_test.rb
@@ -32,7 +32,7 @@ module ActionController
 
         expected = {
           data: {
-            id: assigns(:profile).id.to_s,
+            id: @controller.instance_variable_get(:@profile).id.to_s,
             type: 'profiles',
             attributes: {
               name: 'Name 1',
diff --git a/test/action_controller/explicit_serializer_test.rb b/test/action_controller/explicit_serializer_test.rb
index 00b4db81..8886d07d 100644
--- a/test/action_controller/explicit_serializer_test.rb
+++ b/test/action_controller/explicit_serializer_test.rb
@@ -103,9 +103,9 @@ module ActionController
           {
             'title' => 'New Post',
             'body' => 'Body',
-            'id' => assigns(:post).id,
+            'id' => @controller.instance_variable_get(:@post).id,
             'comments' => [{ 'id' => 1 }, { 'id' => 2 }],
-            'author' => { 'id' => assigns(:author).id }
+            'author' => { 'id' => @controller.instance_variable_get(:@author).id }
           }
         ]
 
diff --git a/test/action_controller/serialization_test.rb b/test/action_controller/serialization_test.rb
index 196d8493..8ffdc100 100644
--- a/test/action_controller/serialization_test.rb
+++ b/test/action_controller/serialization_test.rb
@@ -163,7 +163,7 @@ module ActionController
         end
         expected = {
           data: {
-            id: assigns(:profile).id.to_s,
+            id: @controller.instance_variable_get(:@profile).id.to_s,
             type: 'profiles',
             attributes: {
               name: 'Name 1',
@@ -246,7 +246,7 @@ module ActionController
         expected = {
           data: [
             {
-              id: assigns(:profiles).first.id.to_s,
+              id: @controller.instance_variable_get(:@profiles).first.id.to_s,
               type: 'profiles',
               attributes: {
                 name: 'Name 1',
@@ -269,7 +269,7 @@ module ActionController
         expected = {
           data: [
             {
-              id: assigns(:profiles).first.id.to_s,
+              id: @controller.instance_variable_get(:@profiles).first.id.to_s,
               type: 'profiles',
               attributes: {
                 name: 'Name 1',
diff --git a/test/cache_test.rb b/test/cache_test.rb
index ce208ddf..9dba8d70 100644
--- a/test/cache_test.rb
+++ b/test/cache_test.rb
@@ -102,13 +102,13 @@ module ActiveModelSerializers
 
       render_object_with_cache(uncached_author)
       key = "#{uncached_author_serializer.class._cache_key}/#{uncached_author_serializer.object.id}-#{uncached_author_serializer.object.updated_at.strftime("%Y%m%d%H%M%S%9N")}"
-      key = "#{key}/#{adapter.cached_name}"
+      key = "#{key}/#{adapter.cache_key}"
       assert_equal(uncached_author_serializer.attributes.to_json, cache_store.fetch(key).to_json)
     end
 
     def test_default_cache_key_fallback
       render_object_with_cache(@comment)
-      key = "#{@comment.cache_key}/#{adapter.cached_name}"
+      key = "#{@comment.cache_key}/#{adapter.cache_key}"
       assert_equal(@comment_serializer.attributes.to_json, cache_store.fetch(key).to_json)
     end
 
@@ -139,9 +139,9 @@ module ActiveModelSerializers
       Timecop.freeze(Time.current) do
         render_object_with_cache(@post)
 
-        key = "#{@post.cache_key}/#{adapter.cached_name}"
+        key = "#{@post.cache_key}/#{adapter.cache_key}"
         assert_equal(@post_serializer.attributes, cache_store.fetch(key))
-        key = "#{@comment.cache_key}/#{adapter.cached_name}"
+        key = "#{@comment.cache_key}/#{adapter.cache_key}"
         assert_equal(@comment_serializer.attributes, cache_store.fetch(key))
       end
     end
@@ -152,9 +152,9 @@ module ActiveModelSerializers
         render_object_with_cache(@post)
 
         # Check if it cached the objects separately
-        key = "#{@post.cache_key}/#{adapter.cached_name}"
+        key = "#{@post.cache_key}/#{adapter.cache_key}"
         assert_equal(@post_serializer.attributes, cache_store.fetch(key))
-        key = "#{@comment.cache_key}/#{adapter.cached_name}"
+        key = "#{@comment.cache_key}/#{adapter.cache_key}"
         assert_equal(@comment_serializer.attributes, cache_store.fetch(key))
 
         # Simulating update on comments relationship with Post
@@ -166,9 +166,9 @@ module ActiveModelSerializers
         render_object_with_cache(@post)
 
         # Check if the the new comment was cached
-        key = "#{new_comment.cache_key}/#{adapter.cached_name}"
+        key = "#{new_comment.cache_key}/#{adapter.cache_key}"
         assert_equal(new_comment_serializer.attributes, cache_store.fetch(key))
-        key = "#{@post.cache_key}/#{adapter.cached_name}"
+        key = "#{@post.cache_key}/#{adapter.cache_key}"
         assert_equal(@post_serializer.attributes, cache_store.fetch(key))
       end
     end
@@ -184,7 +184,7 @@ module ActiveModelSerializers
       hash = render_object_with_cache(@location)
 
       assert_equal(hash, expected_result)
-      key = "#{@location.cache_key}/#{adapter.cached_name}"
+      key = "#{@location.cache_key}/#{adapter.cache_key}"
       assert_equal({ place: 'Nowhere' }, cache_store.fetch(key))
     end
 
@@ -276,7 +276,7 @@ module ActiveModelSerializers
 
     def test_uses_file_digest_in_cache_key
       render_object_with_cache(@blog)
-      key = "#{@blog.cache_key}/#{adapter.cached_name}/#{::Model::FILE_DIGEST}"
+      key = "#{@blog.cache_key}/#{adapter.cache_key}/#{::Model::FILE_DIGEST}"
       assert_equal(@blog_serializer.attributes, cache_store.fetch(key))
     end
 
@@ -291,7 +291,7 @@ 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.cached_name}" }
+      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+} }
     end
@@ -306,13 +306,13 @@ module ActiveModelSerializers
         include_directive = ActiveModelSerializers.default_include_directive
         cached_attributes = ActiveModel::Serializer.cache_read_multi(serializer, attributes, include_directive)
 
-        assert_equal cached_attributes["#{@comment.cache_key}/#{attributes.cached_name}"], Comment.new(id: 1, body: 'ZOMG A COMMENT').attributes
-        assert_equal cached_attributes["#{@comment.post.cache_key}/#{attributes.cached_name}"], Post.new(id: 'post', title: 'New Post', body: 'Body').attributes
+        assert_equal cached_attributes["#{@comment.cache_key}/#{attributes.cache_key}"], Comment.new(id: 1, body: 'ZOMG A COMMENT').attributes
+        assert_equal cached_attributes["#{@comment.post.cache_key}/#{attributes.cache_key}"], Post.new(id: 'post', title: 'New Post', body: 'Body').attributes
 
         writer = @comment.post.blog.writer
         writer_cache_key = writer.cache_key
 
-        assert_equal cached_attributes["#{writer_cache_key}/#{attributes.cached_name}"], Author.new(id: 'author', name: 'Joao M. D. Moura').attributes
+        assert_equal cached_attributes["#{writer_cache_key}/#{attributes.cache_key}"], Author.new(id: 'author', name: 'Joao M. D. Moura').attributes
       end
     end
 
diff --git a/test/support/rails_app.rb b/test/support/rails_app.rb
index 16e776c7..0bbae1fe 100644
--- a/test/support/rails_app.rb
+++ b/test/support/rails_app.rb
@@ -22,15 +22,6 @@ ActionController::TestCase.class_eval do
   def setup
     @routes = Routes
   end
-
-  # For Rails5
-  # https://github.com/rails/rails/commit/ca83436d1b3b6cedd1eca2259f65661e69b01909#diff-b9bbf56e85d3fe1999f16317f2751e76L17
-  def assigns(key = nil)
-    warn "DEPRECATION: Calling 'assigns(#{key})' from #{caller[0]}"
-    assigns = {}.with_indifferent_access
-    @controller.view_assigns.each { |k, v| assigns.regular_writer(k, v) }
-    key.nil? ? assigns : assigns[key]
-  end
 end
 
 # ActiveRecord::Migrator.migrations_paths = [File.expand_path("../../test/dummy/db/migrate", __FILE__)]