unique_constraint.rb

module UniqueConstraint
  def self.included(model)
    model.extend(ClassMethods)
  end

  module ClassMethods
    def unique_constraint_violation_message
      "#{name} must be unique"
    end

    def set_unique_constraint_violation_message(message)
      define_attr_method :unique_constraint_violation_message, message
    end
    alias :unique_constraint_violation_message= :set_unique_constraint_violation_message
  end

  # override ActiveRecord::Base
  def save!(*args, &block)
    super(*args, &block)
  rescue ActiveRecord::RecordNotUnique
    raise $!.exception(self.class.unique_constraint_violation_message)
  end
end