atk4\data\Model::addExpression PHP Method

    public function addExpression($name, $defaults)
    {
        if (!is_array($defaults)) {
            $defaults = ['expr' => $defaults];
        } elseif (isset($defaults[0])) {
            $defaults['expr'] = $defaults[0];
            unset($defaults[0]);
        }
        $c = $this->_default_class_addExpression;
        return $this->add(new $c($defaults), $name);
    }

 public function testReloading()
 {
     $a = ['math' => [['a' => 2, 'b' => 2]]];
     $this->setDB($a);
     $db = new Persistence_SQL($this->db->connection);
     $m = new Model($db, 'math');
     $m->addFields(['a', 'b']);
     $m->addExpression('sum', '[a] + [b]');
     $m->load(1);
     $this->assertEquals(4, $m['sum']);
     $m->save(['a' => 3]);
     $this->assertEquals(5, $m['sum']);
     $this->assertEquals(9, $m->unload()->save(['a' => 4, 'b' => 5])->get('sum'));
     $this->setDB($a);
     $m = new Model($db, ['math', 'reload_after_save' => false]);
     $m->addFields(['a', 'b']);
     $m->addExpression('sum', '[a] + [b]');
     $m->load(1);
     $this->assertEquals(4, $m['sum']);
     $m->save(['a' => 3]);
     $this->assertEquals(4, $m['sum']);
     $this->assertEquals(null, $m->unload()->save(['a' => 4, 'b' => 5])->get('sum'));
 }