Conditional validation on model dependent on value of model attribute

I'm struggling with validating a postal_code and using a different regex depending on what the country_code attribute is. I tried the following, but it does not work:

class Venue < ActiveRecord::Base
  ...
  attr_accessible :postal_code
  attr_accessible :country_code
  ...
  validates_presence_of     :country_code
  validates_length_of       :country_code,  :maximum => 2,    :allow_nil => true
  validates_inclusion_of    :country_code,  :in => %w( US CA AU GB )
  validates_presence_of     :postal_code
  validates_length_of       :postal_code,   :maximum => 10,   :allow_nil => true

  validates_format_of       :postal_code,   :with => %r{^\d{4}$},
                                            :message => "should be in the format 1111",
                                            :allow_nil => true,
                                            :if => :country_code == 'AU'
  validates_format_of       :postal_code,   :with => %r{^\d{5}([\-]\d{4})?$},
                                            :message => "should be in the format 11111 or 11111-1111",
                                            :allow_nil => true,
                                            :if => :country_code == 'US'
  validates_format_of       :postal_code,   :with =&g开发者_运维百科t;%r{^[ABCEGHJKLMNPRSTVXY]{1}\d{1}[A-Z]{1} *\d{1}[A-Z]{1}\d{1}$},
                                            :message => "should be in the format A1A 1A1 (try making letters capitalized)",
                                            :allow_nil => true,
                                            :if => :country_code == 'CA'

  # ridculously long (but thorough) regex courtesy of Wikipedia (http://en.wikipedia.org/wiki/Postcodes_in_the_United_Kingdom)
  validates_format_of       :postal_code,   :with => %r{^(GIR 0AA)|(((A[BL]|B[ABDHLNRSTX]?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[HNX]?|F[KY]|G[LUY]?|H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]|R[GHM]|S[AEGKLMNOPRSTY]?|T[ADFNQRSW]|UB|W[ADFNRSV]|YO|ZE)[1-9]?[0-9]|((E|N|NW|SE|SW|W)1|EC[1-4]|WC[12])[A-HJKMNPR-Y]|(SW|W)([2-9]|[1-9][0-9])|EC[1-9][0-9]) [0-9][ABD-HJLNP-UW-Z]{2})$},
                                            :message => "invalid postal code (try making letters capitalized)",
                                            :allow_nil => true,
                                            :if => :country_code == 'GB'

Am I totally off the reservation on this, or is it just the format of the :if options that are hosed up?

For reference, the :if => parameter accepts a method rather than a condition, you can for example do

validate_presence_of :foo, :if => :country_is_gb?

def country_is_gb?
  country_code == 'GB'
end

But as mentioned, lengthy or complex validations are best done in a seperate custom validator.

Take a look at this screencast http://asciicasts.com/episodes/211-validations-in-rails-3 I think its just about time to write a custom validator function for your postal code.