controller.php

    public function actionTest()
    {
        $n = new \common\models\Campaign;
        $n->url = 'blargh';
        $n->save(false);
    }

model.php

    public function behaviors()
    {
        return [
            'timestamp' => [
                'class' => TimestampBehavior::className(),
            ],
            [
                'class' => \yii\behaviors\SluggableBehavior::className(),
                'attribute' => 'url',
                'slugAttribute' => 'slug',
                'immutable' => true,
                'ensureUnique'=>true,
            ],
        ];
    }