custom_validator.rb

ActiveRecord::Base.class_eval do

  def self.validates_inlcusion_of(*attr_names)
    configuration = { :minumum => 1 }
    configuration.update(attr_names.extract_options!)
    validates_each(attr_names, configuration) do |record, attr_name, value|
      unless value.nil?
        record.errors.add(attr_name, "must contain at least #{configuration[:minimum]}") if value.size < configuration[:minimum].to_i
      end
    end
  end
end

some_model.rb

class SomeModel < ActiveRecord::Base
  validates_inlcusion_of :choices, :minimum => 2
end