LazyGenerate module lets you lazily generate properties for ActiveRecord

lazy_generate.rb

# LazyGenerate makes it easy to make new ActiveRecord objects with accessors 
# that are generated and saved lazily
# 
# To use, add this to your ActiveRecord class:
#   extend LazyGenerate
#   lazy_generate :my_lazy_property, :generate_lazy_property
#
# Side effect: Save is called on the object upon generation.
#
module LazyGenerate
  def lazy_generate(column, generate_method)
    class_eval <<-RUBY, __FILE__, __LINE__+1
      def #{column}_with_laziness
        if existing_value = #{column}_without_laziness
          existing_value
        else
          self.#{column} = new_value = #{generate_method}
          save
          new_value
        end
      end
      alias_method_chain :#{column}, :laziness
    RUBY
  end
end

lazy_generate_spec.rb

require 'spec_helper'

describe LazyGenerate do
  describe '.lazy_generate' do
    class LazyGenerateTest
      extend LazyGenerate
      attr_accessor :value
      def generator
        'xyz987'
      end
      def save; end
      lazy_generate :value, :generator
    end
    context 'with an existing value' do
      it 'should pass the value if already set' do
        obj = LazyGenerateTest.new
        obj.value = 'abc123'
        obj.value.should eql 'abc123'
      end
    end
    context 'with an existing value' do
      it 'should generate the value if not already set' do
        LazyGenerateTest.new.value.should eql 'xyz987'
      end
    end
  end
end