start implementing json_api adapter to understand how associations should work

test/adapter/json_api_adapter_test.rb

@@ -0,0 +1,31 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    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.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)
+          @adapter = ActiveModel::Serializer::Adapter::JsonApiAdapter.new(@post_serializer)
+        end
+
+        def test_includes_comment_ids
+          assert_equal(['1', '2'], @adapter.serializable_hash[:comments])
+        end
+      end
+    end
+  end
+end
+