Here is a more complex "not in" query, using a subquery in rails 4 using squeel. Of course very slow compared to the equivalent sql, but hey, it works.
scope :translations_not_in_english, ->(calmapp_version_id, language_iso_code){
join_to_cavs_tls_arr(calmapp_version_id).
joins_to_tl_arr.
where{ tl1.iso_code == 'en' }.
where{ cavtl1.calmapp_version_id == my{calmapp_version_id}}.
where{ dot_key_code << (Translation.
join_to_cavs_tls_arr(calmapp_version_id).
joins_to_tl_arr.
where{ tl1.iso_code == my{language_iso_code} }.
select{ "dot_key_code" }.all)}
}
The first 2 methods in the scope are other scopes which declare the aliases cavtl1 and tl1. << is the not in operator in squeel.
Hope this helps someone.