How do you query an Element by a custom field in Craft 3? 
My first try, according to [craftcms/docs](https://github.com/craftcms/docs/blob/master/en/element-queries.md) was: 

    $country = Entry::find()
                    ->section('countries')
                    ->where(['LIKE', 'code', 'CA'])->one()

 But this throws an exception. Here `code` is a custom text field. I've also tried [some answers](https://craftcms.stackexchange.com/questions/8587/filter-users-by-a-custom-field) on stack exchange and many other combinations but they're for craft 2 and neither they work.