Rails - Feb 2009 - 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
  self.ar_transactions.sum(:balance)
end


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
end

def total
  self.details.sum(:amount)
end

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