Associations, calculate and virtual attributes


I am considering how to approach the problem of providing client
balances. I have considered treating balances as virtual attributes in
some fashion approximating this:

class Client < AR

has_many :ar_transactions, :class => ‘Transaction’,
:conditions => { :type => ‘ar’,
:valid? = true }

def balance

I am also thinking, at the moment, that the balance of a transaction is
similarly a virtual attribute that might look like this:

class Transaction < AR

has_many :details, :class => ‘TransactionDetail’,
:conditions => { :valid? = true, …

has_many :offsets, :class => ‘TransactionJournal’,
:conditions => { …

I do not account for the separate treatment of debit and credit items

in this example

def balance
offset = self.offsets.sum(:amount)
total - offset

def total

Is this the way I should do this? Is there a better way?