preload.rb

pry(main)> topics=Topic.distinct.joins('LEFT OUTER JOIN lessons ON lessons.topic_id = topics.id AND lessons.status = 1').preload(:lessons).where(course_id: 1)
  Topic Load (0.8ms)  SELECT DISTINCT "topics".* FROM "topics" LEFT OUTER JOIN lessons ON lessons.topic_id = topics.id AND lessons.status = 1 WHERE "topics"."course_id" = $1  ORDER BY "topics"."position" DESC, "topics"."id" ASC  [["course_id", 1]]
  Lesson Load (0.5ms)  SELECT "lessons".* FROM "lessons" WHERE "lessons"."topic_id" IN (1, 2, 3, 4, 5, 18, 6, 23, 24)  ORDER BY "lessons"."position" DESC, "lessons"."id" ASC

[109] pry(main)> topics.collect { |topic| [topic.title, topic.lessons.length] }
=> [["Introduction", 5],
 ["Basic Command Line Usage", 5],
 ["Customizing Terminal and Command Prompts", 3],
 ["Advanced Finding", 3],
 ["Networking", 4],
 ["Single Topic", 0],
 ["Tools", 3],
 ["Test", 0],
 ["No lesson module", 0]]