Is there a way to select records by the size of a string field using ARel? For example if I have a bunch of users, can I select all user's whose first names are 3 or fewer characters? I can do something like the following with strings
User.where('LENGTH("users"."firstname") < 3')
I'd like to be able to do something like
User.where(User.arel_table[:firstname].length.lt(3)
Unfortunately no. That being said, the reason you try to use Arel functionality is to avoid database dependent SQL functionality (e.g. Postgres and Mysql have different date handling functions). LENGTH(), on the other hand, is a standard SQL function, and you won't have problems with portability of that query.
tl;dr - Using custom SQL for everything is bad practice. Using custom SQL strategically is totally fine.