dynamic_inclusion_validator.rb

class DynamicInclusionValidator < ActiveModel::EachValidator
  def check_validity!
    raise ArgumentError, "A proc which result has the method include? is required as the " <<
        ":in option of the configuration hash" unless options[:in].respond_to?(:call)
  end

  def validate_each(record, attribute, value)
    unless options[:in].bind(record).call.include?(value)
      record.errors.add(attribute, :inclusion, options.except(:in).merge!(:value => value))
    end
  end
end

# Usage

class Ranking < ActiveRecord::Base
  RANKING_TYPES = {
    :official => 0,
    :public => 1,
    :private => 2,
  }

  belongs_to :user

  validates :ranking_type, :dynamic_inclusion => {:in => Proc.new{ user.present?  ? user.ranking_types.values : Ranking::RANKING_TYPES.values} }
end