resolve_attributes

# File activerecord/lib/active_record/validations/uniqueness.rb, line 97
      def resolve_attributes(record, attributes)
        attributes.flat_map do |attribute|
          reflection = record.class._reflect_on_association(attribute)

          if reflection.nil?
            attribute.to_s
          elsif reflection.polymorphic?
            [reflection.foreign_key, reflection.foreign_type]
          else
            reflection.foreign_key
          end
        end
      end