validate_belongs_to_association

# File activerecord/lib/active_record/autosave_association.rb, line 343
      def validate_belongs_to_association(reflection)
        association = association_instance_get(reflection.name)
        record      = association && association.reader
        return unless record && (record.changed_for_autosave? || custom_validation_context?)

        begin
          @validating_belongs_to_for ||= {}
          @validating_belongs_to_for[association] = true
          association_valid?(association, record)
        ensure
          @validating_belongs_to_for[association] = false
        end
      end