diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 106c5e71..1bfdf825 100644
--- a/lib/active_model/serializer.rb
+++ b/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
diff --git a/lib/active_model/serializer/adapter/json_api_adapter.rb b/lib/active_model/serializer/adapter/json_api_adapter.rb
index 5f5df719..14094417 100644
--- a/lib/active_model/serializer/adapter/json_api_adapter.rb
+++ b/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
diff --git a/lib/active_model/serializer/array_serializer.rb b/lib/active_model/serializer/array_serializer.rb
index 13ab08c7..c2751f4b 100644
--- a/lib/active_model/serializer/array_serializer.rb
+++ b/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
diff --git a/test/adapter/json_api_adapter_test.rb b/test/adapter/json_api_adapter_test.rb
index c49a90c1..dc09e397 100644
--- a/test/adapter/json_api_adapter_test.rb
+++ b/test/adapter/json_api_adapter_test.rb
@@ -5,24 +5,19 @@ module ActiveModel
     class Adapter
       class JsonApiTest < Minitest::Test
         def setup
-          @post = ::Model.new(title: 'New Post', body: 'Body')
-          @first_comment = ::Model.new(id: 1, body: 'ZOMG A COMMENT')
-          @second_comment = ::Model.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
+          @post = Post.new(title: 'New Post', body: 'Body')
+          @first_comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
+          @second_comment = Comment.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
           @post.comments = [@first_comment, @second_comment]
           @first_comment.post = @post
           @second_comment.post = @post
 
-          @post_serializer_class = def_serializer do
-            attributes :title, :body
-            has_many :comments
-          end
-
-          @post_serializer = @post_serializer_class.new(@post)
+          @post_serializer = PostSerializer.new(@post)
           @adapter = ActiveModel::Serializer::Adapter::JsonApiAdapter.new(@post_serializer)
         end
 
         def test_includes_comment_ids
-          assert_equal(['1', '2'], @adapter.serializable_hash[:comments])
+          assert_equal([1, 2], @adapter.serializable_hash[:comments])
         end
       end
     end
diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index 8876a73d..8359fcb7 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -5,12 +5,16 @@ class Model
 
   def read_attribute_for_serialization(name)
     if name == :id || name == 'id'
-      object_id
+      id
     else
       @attributes[name]
     end
   end
 
+  def id
+    @attributes[:id] || @attributes['id'] || object_id
+  end
+
   def method_missing(meth, *args)
     if meth.to_s =~ /^(.*)=$/
       @attributes[$1.to_sym] = args[0]
@@ -28,3 +32,18 @@ end
 class ProfileSerializer < ActiveModel::Serializer
   attributes :name, :description
 end
+
+Post = Class.new(Model)
+Comment = Class.new(Model)
+
+PostSerializer = Class.new(ActiveModel::Serializer) do
+  attributes :title, :body, :id
+
+  has_many :comments
+end
+
+CommentSerializer = Class.new(ActiveModel::Serializer) do
+  attributes :id, :body
+
+  belongs_to :post
+end
diff --git a/test/serializers/associations_test.rb b/test/serializers/associations_test.rb
index 45fbd8d1..56603b2c 100644
--- a/test/serializers/associations_test.rb
+++ b/test/serializers/associations_test.rb
@@ -3,10 +3,6 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class AssocationsTest < Minitest::Test
-      def def_serializer(&block)
-        Class.new(ActiveModel::Serializer, &block)
-      end
-
       class Model
         def initialize(hash={})
           @attributes = hash
@@ -26,19 +22,7 @@ module ActiveModel
           end
         end
       end
-      Post = Class.new(Model)
-      Comment = Class.new(Model)
-      PostSerializer = Class.new(ActiveModel::Serializer) do
-        attributes :title, :body
 
-        has_many :comments
-      end
-
-      CommentSerializer = Class.new(ActiveModel::Serializer) do
-        attributes :id, :body
-
-        belongs_to :post
-      end
 
       def setup
         @post = Post.new({ title: 'New Post', body: 'Body' })