Yii2 How to create a validation rule on two unique attributes?

Question

Yii

I do like this...

but the validation skips the entry and I get an Exception

Error Info: Array
(
    [0] => 23000
    [1] => 1062
    [2] => Duplicate entry '1-4' for key 'a1_a2'
)

how to make it check for uniqueness on two fields a1 and a2 at once?