# Not good

class User < ActiveRecord::Base
  validates :appropriate_content

  def appropriate_content
    unless # some validation check on name
      self.errors.add(:name, 'is inappropriate')
    end
  end
end

# Better
require 'appropriate_validator'
class User < ActiveRecord::Base
  validate :name, appropriate: true
end

# /lib/appropriate_validator.rb
class AppropriateValidator < ActiveRecord::EachValidator
  def validate_each(record, attr, value)
    unless # some validation check on value
      record.errors.add(:attr, "is inappropriate")
    end
  end
end