Cakephp搜索插件filterArgs（如果在数组中则为else）

I want to make an if else in filterArgs (in Model) as follow:

public $filterArgs = array(
            array('name' => 'to', 'type' => 'value' => 'Product.regular_price' ),
        );

I want the field to change base on the condition.

if sale = 1, field is promo_price, else field is regular_price

I already try below code (but unsuccessful):

'field'=> 'Product.sale' => 1 ? 'Product.promo_price >=' : 'Product.regular_price >='

Can someone please help me. Thanks alot in advance!

I would create a virtualField in my Product Model

$virtualFields = array 
(
    'my_price' => 'IF(Product.sale = 1, Product.promo_price, Product.regular_price)';
)

public $filterArgs = array
(
    'my_price' => array('type' => 'value'),
);