How to get the sum of items using relays in Yii2?

Question

Yii

class Customer
{
    /** @var $totalHouses total Houses */
    public $totalHouses;

    /** @var $houses array of Houses */
    public $houses;

    /** @var $sum total sum of Houses */
    public $sum;
}

class House
{
    /** @var $District District */
    public $District;

    /** @var $cost float */
    public $cost;

    /** @var $customer Customer|null */
    public $Customer;
}

class District
{
    /** @var $totalHouses total Houses */
    public $totalHouses;

    /** @var $houses array of Houses */
    public $houses;

    /** @var $sum total sum of Houses */
    public $sum;
}

links:
houses.customer_id = customer.id
houses.district_id = district.id
how to get the total amount of houses from a buyer?
how to find out the total amount of houses in the area?
how to find out the total number of houses in the area?

Answer 1

So here it is:
sum + yii2 relation
How get sum of field in the related table in one q...
The only thing that is not very good is that "greedy" loading does not work!