Unfortunately SQL leaks through Arel's abstractions and make it behave in surprising ways. Arel falls short of achieving the goal of being able to modularise and compose queries. The same is true of nearly every SQL connectivity library (or so this article claims: http://www.try-alf.org/blog/2013-10-21-relations-as-first-cl...).
> Unfortunately SQL leaks through Arel's abstractions
This is voluntary. Arel allows one to write SQL almost as is, only with native types and features that make it possible to compose and aggregate queries. It's not a 1:1 mapping but it's not a blackbox abstraction either since the goal is to finely control your SQL. Of note, Arel is neither ActiveRecord nor ActiveRecord::Relation (which is what you get when you use #all, #where and scopes, sadly often mistaken for Arel)
The article you linked to is wrong, Arel is perfectly able to compose projections and joins, see this gist[0] implementing what the author demands.
Unfortunately SQL leaks through Arel's abstractions and make it behave in surprising ways. Arel falls short of achieving the goal of being able to modularise and compose queries. The same is true of nearly every SQL connectivity library (or so this article claims: http://www.try-alf.org/blog/2013-10-21-relations-as-first-cl...).