model:
public class SomeModel
{
    public $id; //Not nullable
    public $fieldA; //Not nullable
    public $fieldB; //Not nullable
    .
    .
    .
controller:
public class SomeController
{
    public function SomeAction()
    {
        $someModel = SomeModel::findFirst(...);
        $someModel->fieldB = 123;
        $someModel->update();
        //if the original value of fieldA in db is empty string, it will raise an error here
        //fieldA is required
        .
        .
        .