class RuboCop::Cop::Rails::UniqueValidationWithoutIndex

def column_names(node, uniqueness_part)
  arg = node.first_argument
  return unless arg.type?(:str, :sym)
  ret = [arg.value]
  names_from_scope = column_names_from_scope(uniqueness_part)
  ret.concat(names_from_scope) if names_from_scope
  ret = ret.flat_map do |name|
    klass = class_node(node)
    resolve_relation_into_column(
      name: name.to_s,
      class_node: klass,
      table: schema.table_by(name: table_name(klass))
    )
  end
  ret.include?(nil) ? nil : ret.to_set
end