Fix for unpersisted has_one's through being inaccessible in Rails.

active_record_unpersisted_has_one_fix.rb

# FIXME Rails has-one through is nil on unpersisted objects (bug)
#       https://github.com/rails/rails/issues/33155
module ActiveRecordUnpersistedHasOneFix
  extend ActiveSupport::Concern
  
  # Define utility methods
  class << self
    # @return [Array<String>, nil]
    def association_path(model_class, association)
      path = []
      reflection = model_class.reflections.fetch(association.to_s)

      while reflection.through_reflection? && reflection != reflection.source_reflection
        path << reflection.through_reflection.name
        reflection = reflection.source_reflection
      end

      if path.length > 0
        path << reflection.name
        path
      else
        nil
      end
    end

    def read_association(model_instance, association)
      path = association_path(model_instance.class, association)

      path && path.reduce(model_instance) { |result, path_segment| result.try(path_segment) }
    end
  end
  

  class_methods do
    def has_one(*args, **kwargs, &block)
      super(*args, **kwargs, &block).tap do

        association_name = args.first

        # This anonymous module is used to define all the has_one's per class
        @unpersisted_has_one_fix_module ||= Module.new do
          extend ActiveSupport::Concern
        end

        include @unpersisted_has_one_fix_module

        @unpersisted_has_one_fix_module.class_eval <<-METHODS, __FILE__, __LINE__ + 1
          def #{association_name}
            if persisted?
              super
            else
              super || ::ActiveRecordUnpersistedHasOneFix.read_association(self, "#{association_name}")
            end
          end
        METHODS
      end
    end

  end
end