yeah, it's a ```find_in_batches``` thing:
if u dont add ```:id```,  you get:
```
RuntimeError:
       Primary key not included in the custom select clause
```
https://github.com/rails/rails/blob/13d2696c10726afecd393753fcac88c5a9907d8c/activerecord/lib/active_record/relation/batches.rb#L123