gistfile1.rb

def filter_by_highest_ranked_user_coaches
  coach_user_ids_string = coach_user_ids.to_s.gsub('[', '').gsub(']', '')

  highest_ranked_coach_ids = ActiveRecord::Base.connection.execute(<<-TEXT
    SELECT DISTINCT on (user_id) user_id, id
    FROM coaches c
    WHERE coachup_ranking_score =
      (SELECT MAX(r.coachup_ranking_score)
        FROM coaches r
        WHERE c.user_id = r.user_id
        AND c.user_id IN (#{coach_user_ids_string}))
    TEXT
  ).values.map { |user_id, coach_id| coach_id }

  @scope.where(coach_id: highest_ranked_coach_ids)
end