[Question] How to update fields with projection?

[matisiekpl]

Hello.
I want to lower case specific field in my collection

DB::collection('offers')->update(['gearbox_type' => ['$toLower' => '$gearbox_type']]);

But, it's not working, beacuse of this error:

The dollar ($) prefixed field '$toLower' in 'gearbox_type.$toLower' is not valid for storage.

How to do it properly?