Yii2: Eagerly selecting calculated column and loading value into model-property - activerecord

Question

I thought I know every aspect of Yii2 in the meantime, but this one gives me headaches.
Situation
Two tables: Client and Billings. The Client-Table holds a regular list of clients. The Billing-table has several entries for each client (1:n).
Problem
I want to fetch a calculated DB-Field together with the row itself and access it via a virtual property of the model.
Key is that it gets calculated and selected together with the row itself. I know I can achieve something similliar with a regular virtual getter calculating the amount...but this is not at the same time as the select itself.
My Plan
In the query-object of the client-model i tried to add an an additional select (addSelect-Method) and give the field an alias. Then I added the alias of this select with the attributes-method of the model. Somehow this didn't work.
My Question
Does someone of you know the right way to achieve this? As this is a very common problem, I can not imagine this beeing too hard. I just somehow can't find the solution.
Sample code:
echo $client->sumOfBillings should output the contents of the corresponding property within the client-model. The contents of this property should be filled when fetching the client-row itself and not at the moment the property gets called.

Answer 1

I actual found the answer myself. Here is how you do it:
Query object
The fetching of all the Yii2-Models is done via their corresponding Query-Object. This object is retrieved via the models find()-Method. If you override this method, you can return your own query-object for that class. In the example above my model looks like this:
class Client extends \yii\db\ActiveRecord
{
//...
public static function find()
{
return new ClientQuery(get_called_class());
}
//...
}
Now within the Query-Objects init()-Method we can add the corresponding additional selects:
public class ClientQuery extends \yii\db\ActiveQuery
{
public function init()
{
parent::init();
//prepare subquery for calculation
$sub = (new Query())
->select('SUM(billing_amount)')
->from('billing')
->where('billing.client_id = client.id');
$this->addSelect(['client.*', 'sumBillings'=>$sub]);
}
}
We are now done with the query-Object. What have we done now? When selecting a client the sum gets calculated and loaded as well. But how do we access it? This was the hard part where I struggeled. The solution lies within the ActiveRecord-class.
Possibilities to populate the model with calculated data
There are several possibilities to load this data into the model-class. To understand what options we have, we can check out the populateRecord($record, $row)-method of the BaseActiveRecord-class:
/**
* Populates an active record object using a row of data from the database/storage.
*
* This is an internal method meant to be called to create active record objects after
* fetching data from the database. It is mainly used by [[ActiveQuery]] to populate
* the query results into active records.
*
* When calling this method manually you should call [[afterFind()]] on the created
* record to trigger the [[EVENT_AFTER_FIND|afterFind Event]].
*
* #param BaseActiveRecord $record the record to be populated. In most cases this will be an instance
* created by [[instantiate()]] beforehand.
* #param array $row attribute values (name => value)
*/
public static function populateRecord($record, $row)
{
$columns = array_flip($record->attributes());
foreach ($row as $name => $value) {
if (isset($columns[$name])) {
$record->_attributes[$name] = $value;
} elseif ($record->canSetProperty($name)) {
$record->$name = $value;
}
}
$record->_oldAttributes = $record->_attributes;
}
As you can see, the method takes the raw-data ($row) and populates the model instance ($record). If the model has either a property or a setter-method with the same name as the calculated field, it will be populated with data.
Final code of Client-Model
This is my final code of the Client-model:
class Client extends \yii\db\ActiveRecord
{
private $_sumBillings;
//...
public static function find()
{
return new ClientQuery(get_called_class());
}
public function getSumBillings()
{
return $this->_sumBillings;
}
protected function setSumBillings($val)
{
$this->_sumBillings = $val;
}
//...
}
The populateRecord()-method will find the setter-method ($record->canSetProperty($name)) and call it to fill in the calculated value. As it is protected, it is otherwise readonly.
Voilà...not that hard actually and definitely useful!