invalid_length_error.rb

class InvalidLengthError
  attr_accessor :min, :max, :current
  private :min=, :max=, :current=

  CODE = "INVALID_LENGTH"
  DESCRIPTION = "Invalid length"

  def initialize(min, max, current)
    self.min = min
    self.max = max
    self.current = current
  end

  def code
    CODE
  end

  def description
    DESCRIPTION
  end

  def empty?
    false
  end
end

user.rb

require 'invalid_length_error'

class User < ActiveRecord::Base

  validate :login_length

  private

  def login_length
    return if login.size >= 2 && login.size <= 10
    errors.add(:login, InvalidLengthError.new(2, 10, login.size))
  end

end

users_helper.rb

module UsersHelper

  def error_message(error)
    return error if error.is_a?(String)               # Message is just a string, we can't even try to do something nice with it
    method = error.code.to_s.downcase + "_error_message"
    return send(method, error) if respond_to?(method) # We have a special way of displaying such error
    return error.description                          # Just display the description from model
  end

  def invalid_length_error_message(error)
    "length of this text is invalid. Minimum is <strong>#{error.min}</strong> and maximum is <strong>#{error.max}</strong>".html_safe
  end

end