Calculate using Relational Query

I am creating a Yii app which calculate a grant total from subtotal.

The customer will have a dropdown like

wedding attire-10000->item1 and i have here a subtotal

reception-10000->item2 and i have here a subtotal

In this case the user "selected" wedding attire and reception. All i want to do is to add those item.

I also want to limit those dropdownlist.

The things is, can i calculate that using their ID?

I use relational query 'the BELONGS_TO".Hot would i do that?Help me Please.

Can U please explain this a bit more ?

I simply want to add those items,I have a

‘weddingattireRelation’=>array(self::BELONGS_TO, ‘Weddingattire’, ‘weddingattire_id’),

‘receptionRelation’=>array(self::BELONGS_TO, ‘Reception’, ‘reception_id’),

I put those items in other _form and I want to calculate the grand total through their subtotal.

define a total variable and loop thru the records and add the subtotal to that total should be straightforward