gistfile1.rb

class Account < ActiveRecord::Base
  has_many :balance_transfers
  has_many :orders

  def purchase_order(description, cost)
    begin
      transaction do
        balance_transfers.create! :change=>cost
        orders.create! :description=>description
      end
    rescue Exception => ex
      p "rolling back #{ex.message}"
    end
  end
end

class BalanceTransfer < ActiveRecord::Base
  belongs_to :account
  validates_presence_of :account
  
  before_save :update_account_balance
  
  def update_account_balance
    if account.balance < change
      errors.add_to_base("not enough funds")
      raise "not enough points"
    else
      account.update_attribute(:balance,(account.balance - change))
    end

  end    
end

class Order < ActiveRecord::Base
  belongs_to :account
  validates_presence_of :account
  validates_presence_of :description
end

class AccountTest < ActiveSupport::TestCase

  self.use_transactional_fixtures = false

  def setup
    Account.delete_all
    Order.delete_all
    BalanceTransfer.delete_all
  end

  test "enough funds" do
    account=Account.create :name=>"test", :balance=>100
    order=account.purchase_order "stroberi",50
    account.reload
    assert_equal 50,account.balance
    assert_equal 1,account.balance_transfers.size
    assert_equal 1,account.orders.size
    assert_equal order.description, "stroberi"
  end
  
  test "not enough funds" do
    account=Account.create! :name=>"test", :balance=>10
    account.purchase_order "stroberi",50
    account.reload
    assert_equal 10,account.balance 
    assert_equal 0,account.balance_transfers.size
    assert_equal 0,account.orders.size
  end
    
  test "invalid order description" do
    account=Account.create :name=>"test", :balance=>100
    account.purchase_order nil,50
    account.reload
    
    # no point txs should be created
    assert_equal 0, account.balance_transfers.count
    assert_equal 0, account.orders.count
    assert_equal 100,account.balance
    
  end

end