gistfile1.sql

SELECT tappables.id,
          max(case
              when tappables.type = 'Survey'
                then case
                  when (not exists survey_responses) -- pseudo-code
                    then now() - random() * interval '1 day'
                  else
                    max(survey_responses.created_at)
                  end
              when taps.user_id = :user_id
                then taps.created_at
              else
                taps.skewed_created_at
              end) as last_tap
        FROM tappables
        join taps on taps.tappable_id = tappables.id
        join survey_responses as sr
        on sr.survey_id = tappables.id
        and sr.user_id = :user_id
        group by tappables.id
        order by last_tap desc"