gistfile1.rb

require 'rubygems'
require 'dm-core'

DataMapper.setup(:default,
  :adapter => 'mysql',
  :host => 'localhost', 
  :username => 'root',
  :database => 'dm_core_test',
  :encoding => 'utf8'
)

DataObjects::Mysql.logger = DataObjects::Logger.new(STDOUT, :debug)

class Rate
  include DataMapper::Resource
  
  property :id,   Serial
  property :name, String
  property :rate, BigDecimal
  
  has n, :postings
end

class Posting
  include DataMapper::Resource
  
  property :id, Serial
  property :date, Date
  property :amount, BigDecimal, :scale => 2, :precision => 10

  belongs_to :rate
  has n, :children, :model => Posting, :child_key => [:parent_id]
  
  before :save do
    if attribute_dirty?(:rate_id)
      children.destroy if saved?
      children.new :date => date, :amount => 20
    end
  end
end

Rate.auto_migrate!
Posting.auto_migrate!

r  = Rate.create(:name => "25% Incoming VAT", :rate => 25) # id 1
p1 = Posting.create(:amount => 100.00, :rate_id => 1) # id 1
p2 = Posting.create(:amount => 100.00, :rate_id => 1) # id 3 (a child is created)

# There are now 4 postings saved in the database.
# Then we update p2, changing rate_id. As you can see
# In the before-block, it will try to delete its children
# This deletes ALL postings, including itself.
p2.update(:rate_id => nil)

# Tada, all the postings (including p2 itself) are gone:
puts Posting.all.inspect # => []