How can I add a derived column to my Yii model?

Question

In Yii, I need to add a "derived" column to any resultset from my model. The column doesn't actually exist in the database table.

For example, say I have an Activity model. There are only two types of Activities: (1) Income, or (2) Expense.

If I want to add a column called income_total or expense_total (depending on what type of activity is being accessed), how would I do that?

Here's a working example of an Activity model in Ruby on Rails (I'm basically wondering how to do the same thing, but in Yii):

class Activity < ActiveRecord::Base

  attr_accessible :name, :effective_at, :amount, :category
  scope :incomes,  :conditions => { :category => 'Income'  }
  scope :expenses, :conditions => { :category => 'Expense' }

  def self.incomes_total
    incomes.sum :amount
  end

  def self.expenses_total
    expenses.sum :amount
  end

end

Update 2012-07-01:

The answer provided by Leonardo points to the use of Virtual Attributes, this adds an attribute to each "row" of the resultset that I'm retrieving from the database.

If the Activity model has a BELONGS_TO relationship with Parent, a Parent View may look like the following:

<h2><?php echo $parent->name; ?></h2>
<ul>
  <?php foreach($portfolio->activities as $activity): ?>
    <li>
      <?php echo CHtml::link($activity->name, array('activity/view', 'id' => $activity->id)); ?>
      <?php echo $activity->amount; ?>
      <?php echo $activity->incomes_total; ?>
    </li>
  <?php endforeach; ?>
</ul>

However, it doesn't make sense for this Virtual Attribute to be accessed within the foreach() loop.

These Virtual Attributes that I want provide one "aggregated" value for the whole resultset, so I want to be able access it using $parent->activities->incomes_total, like so:

<h2><?php echo $parent->name; ?></h2>
<?php echo $parent->activities->incomes_total; ?>
<ul>
  <?php foreach($portfolio->activities as $activity): ?>
    <li>
      <?php echo CHtml::link($activity->name, array('activity/view', 'id' => $activity->id)); ?>
      <?php echo $activity->amount; ?>
    </li>
  <?php endforeach; ?>
</ul>

What do I need to do within the Activity model code to achieve this, or should I be thinking about this a different way?

Answer 1

I believe it is pretty similar to Ruby. PHP has magic methods, which in Yii is implemented by CComponent (base class of many including CActiveRecord).

In short you can do this within your model

   class Activity extends CActiveRecord {
     public function getIncome_Total() {
       // ...
     }
   }

And, to access it from controller

$activity = Activity::model->findByPk(1);
$income_total = $activity->income_total;

Answer 2

I don't know how this can be done within the model.

Although it is more processing, an option or workaround is to add another foreach() loop to the View like so:

$incomes_total = 0;
foreach($parent->activities as $activity)
  $incomes_total += $activity->amount;
echo $incomes_total;

Answer 3

As I posted in the other answer You can simply use a stat relation. Specify a STAT relation in Parent model as

'incomes_total'=>array(self::STAT, 'Activity', 'parent_id','select'=>'SUM(amount)','condition'=>"incomes_total.category='Income'")

Similiarly

'expense_total'=>array(self::STAT, 'Activity', 'parent_id','select'=>'SUM(amount)','condition'=>"incomes_total.category='Expense'")

NOTE: Change the attribute names parent_id and amount as in your model