class Category < ActiveRecord::Base
  validates_presence_of :title

  private

  def validate
    errors.add(:description, "is too short") if (description.size < 200)
  end
end

class Category < ActiveRecord::Base
  validates_presence_of :title
  validate :description_length

  def description_length
    errors.add(:description, "is too short") if (description.size < 200)
  end
end

class TitleValidator < ActiveModel::EachValidator
  Titles = ['Mr.', 'Mrs.', 'Dr.']
  def validate_each(record, attribute, value)
    unless Titles.include?(value)
      record.errors[attribute] << 'must be a valid title'
    end
  end
end

class Person
  include ActiveModel::Validations
  attr_accessor :title
  validates :title, :presence => true, :title => true
end

# Or for Active Record

class Person < ActiveRecord::Base
  validates :title, :presence => true, :title => true
end