start implementing json_api adapter to understand how associations should work

lib/active_model/serializer/adapter.rb

@@ -4,6 +4,7 @@ module ActiveModel
       extend ActiveSupport::Autoload
       autoload :SimpleAdapter
       autoload :NullAdapter
+      autoload :JsonApiAdapter
 
       attr_reader :serializer
 

lib/active_model/serializer/adapter/json_api_adapter.rb

@@ -0,0 +1,24 @@
+module ActiveModel
+  class Serializer
+    class Adapter
+      class JsonApiAdapter < Adapter
+        def serializable_hash(options = {})
+          hash = serializer.attributes.each_with_object({}) do |(attr, value), h|
+            h[attr] = value
+          end
+
+          serializer.associations(only: [:id]).each_with_object({}) do |(attr, value), h|
+            case value
+            when ActiveModel::Serializer::ArraySerializer
+              # process has_many association
+            when ActiveModel::Serializer
+              # process belongs_to association
+            else
+              # what?
+            end
+          end
+        end
+      end
+    end
+  end
+end