Containable and Aggregate fields

I am trying to get a SUM of a column of related records through the Containable behavior. 

I've tried a number of variations based on information google turned up but I cannot get it to work:

$accounts = $this->find('all', array(

'contain' => array('Registration' => array('fields' => array('SUM(`credits`)'),

  'group' => array('Registration.account_id')))

));

I soon as I add the "group" part, I get: Model "Registration" is not associated with model "Registration"

Here's what I am trying to do in SQL:

SELECT
  accounts.*,
  SUM(registrations.credits) AS slot_count
FROM accounts
LEFT JOIN registrations
  ON (accounts.id=registrations.account_id)
  AND registrations.attend=1
GROUP BY accounts.id 

Maybe I need to just use that query directly instead of trying to use Containable? 

Thanks for any advice..

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