belongs_to causes unnecessary db hit

test/serializers/associations_test.rb

@@ -126,6 +126,23 @@ def test_associations_custom_keys
         assert expected_association_keys.include? :site
       end
 
+      def test_belongs_to_doesnt_load_record
+        class << @post
+          def blog
+            fail 'should use blog_id'
+          end
+        end
+
+        post_serializer = Class.new(ActiveModel::Serializer) do
+          belongs_to :blog
+        end
+
+        actual = serializable(@post, adapter: :json_api, serializer: post_serializer).as_json
+        expected = { data: { id: 'post', type: 'posts', relationships: { blog: { data: { id: 'blog', type: 'blogs' } } } } }
+
+        assert_equal expected, actual
+      end
+
       class InlineAssociationTestPostSerializer < ActiveModel::Serializer
         has_many :comments
         has_many :comments, key: :last_comments do