Custom model field validation

en.yml

en:
  errors:
      models:
        model_name:
          attributes:
            field_name:
              reserved: Такое имя пользователя зарезервированно

free_alias_validator.rb

class FreeAliasValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)

    # TODO: add all routes
    routes = %(admin)

    if routes.include? value
      record.errors.add(attribute, :reserved, options)
    elsif User.find_by_name value
      record.errors.add(attribute, :reserved, options)
    elsif Category.find_by_alias value
      record.errors.add(attribute, :reserved, options)
    end
  end
end

model.rb

  name_regex = /[-a-zA-Z0-9]+$/ui
  validates :field,
            :presence => true,
            :format => {:with => name_regex, :unless => :field_empty?},
            :uniqueness => {:case_sensitive => false},
            free_alias: true, :on => :create

  private

  def field_empty?
    field.nil? || field.empty?
  end