Use a Railtie to extend Rails

lib/extensions/has_one.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-if defined?(::ActiveRecord)
 ::ActiveRecord::Associations::Builder::HasOne.class_eval do
   # Based on
   # https://github.com/rails/rails/blob/master/activerecord/lib/active_record/associations/builder/collection_association.rb#L50
@@ -17,4 +16,3 @@ if defined?(::ActiveRecord)
     CODE
   end
 end
-end

lib/fast_jsonapi.rb

@@ -2,5 +2,9 @@
 
 module FastJsonapi
   require 'fast_jsonapi/object_serializer'
+  if defined?(::Rails)
+    require 'fast_jsonapi/railtie'
+  elsif defined?(::ActiveRecord)
     require 'extensions/has_one'
   end
+end

lib/fast_jsonapi/railtie.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+require 'rails/railtie'
+
+class Railtie < Rails::Railtie
+  initializer 'fast_jsonapi.active_record' do
+    ActiveSupport.on_load :active_record do
+      require 'extensions/has_one'
+    end
+  end
+end