string_max_length_validator.rb

class StringMaxLengthValidator < ActiveModel::EachValidator
  STRING_MAX_LENGTH = 255
  
  def validate_each(record, attribute, value)
    return if string_length_in_range?(value)

    record.errors.add attribute, :'too_long.other', count: STRING_MAX_LENGTH
  end

  private

  def string_length_in_range?(str)
    str.present? && str.length <= STRING_MAX_LENGTH
  end
end

module ActiveModel
  module Validations
    module HelperMethods
      # Validates that the specified attributes match the string maximum length
      # restrictions supplied.
      #
      # class Person < ActiveRecord::Base
      #   validates :first_name, string_max_length: true
      #   validates_string_max_size_of :middle_name, :last_name
      # end
      def validates_string_max_length_of(*attr_names)
        validates_with StringMaxLengthValidator, _merge_attributes(attr_names)
      end

      alias_method :validates_string_max_size_of, :validates_string_max_length_of
    end
  end
end