troublesome validation

Troublesome validation

class Pledge < ActiveRecord::Base

  validates_presence_of :amount


  def validate_positivity_of(*attr_names)
    configuration = { :message => "Cannot be negative" }
    configuration.update(attr_names.pop) if attr_names.last.is_a?(Hash)
    validates_each attr_names do |m, a, v| m.errors.add(a, configuration[:message]) if v<=0 end
  end

  validate_positivity_of :amount


Results in

undefined method `validate_positivity_of' for #<Class:0xb571d184>