Custom Filters in ActiveAdmin: Filtering a Nil Value with Ransacker and ActiveAdmin
lowell mower

You may try this approach:

filter :application_sent_at_not_null, as: :select

I checked on similar case on models in my application, SQL looks like ( except for inner_join in your case it’s the same ):

SELECT "users".* 
FROM "users"
LEFT OUTER JOIN "students"
ON "students"."user_id" = "users"."id"
WHERE ( "students"."application_sent_at" IS NOT NULL )
ORDER BY "users"."id"