$Ch->find(
'all',
'fields' => array(
'Ch.id','COUNT( DU.d_id ) AS a', 'SUM(DU.d_id) AS b'),
'joins' => array(
array(
'table' => 'du',
'alias' => 'DU',
'type' => 'LEFT',
'conditions' => array('DU.c_id = Ch.id AND MONTH( DU.created ) = MONTH( NOW( )')),
array(
'table' => 'd',
'alias' => 'D',
'type' => 'LEFT',
'conditions' => array('DU.c_id = Ch.id AND MONTH( DU.created ) = MONTH( NOW( )')),
'group' => 'Ch.id');
Я не проверял.Дополнительная информация о Cake JOINs:
http://book.cakephp.org/view/1018/find