First try to implement ArraySerializer

lib/active_model/serializer.rb

@@ -108,12 +108,7 @@ module ActiveModel
       self.class._associations.dup.each_with_object({}) do |(name, value), hash|
         association = object.send(name)
         serializer_class = ActiveModel::Serializer.serializer_for(association)
-        if serializer_class
-          serializer = serializer_class.new(association)
-          hash[name] = serializer
-        else
-          hash[name] = association
-        end
+        hash[name] = serializer_class.new(association)
       end
     end
   end

lib/active_model/serializer/adapter/json_api_adapter.rb

@@ -3,20 +3,21 @@ module ActiveModel
     class Adapter
       class JsonApiAdapter < Adapter
         def serializable_hash(options = {})
-          hash = serializer.attributes.each_with_object({}) do |(attr, value), h|
-            h[attr] = value
-          end
+          hash = serializer.attributes
 
-          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
+          associations = serializer.associations(only: [:id]).each_with_object({}) do |(attr, value), h|
+            h[attr] = case value
+                       when ActiveModel::Serializer::ArraySerializer
+                         value.attributes(options).map do |item|
+                           item.id
+                         end.to_a
+                       when ActiveModel::Serializer
+                         # process belongs_to association
+                       else
+                         # what?
+                       end
           end
+          hash.merge(associations)
         end
       end
     end

lib/active_model/serializer/array_serializer.rb

@@ -4,6 +4,13 @@ module ActiveModel
       def initialize(object)
         @object = object
       end
+
+      def attributes(options = {})
+        object.map do |item|
+          serializer_class = ActiveModel::Serializer.serializer_for(item)
+          serializer_class.new(item)
+        end
+      end
     end
   end
 end