My app has models Person, Photo, Comment. A Comment has_many Photos and a Photo has_many People.
In the console, with a database with some data in it and the ActiveRecord logger sent to STDOUT …
First, to show what’s in the data:
irb(main):003:0> Photo.joins(:comments).order(:id).limit(2).map &:id
D, [2014-08-05T18:19:10.528376 #24027] DEBUG – : Photo Load (20.6ms) SELECT photos.* FROM photos INNER JOIN comments ON comments.photo_id = photos.id ORDER BY photos.id ASC LIMIT 2
=> [1, 1]
Photo.joins(:comments).order(:id).limit(2) returns the same photo twice, because it has two comments. That’s what I want. (This is cut down from a much larger query where this behavior is actually useful.) Now for the problem:
irb(main):001:0> Photo.joins(:comments).includes(:person).order(:id).limit(2).each { |f| puts f.person.username }; nil
D, [2014-08-05T18:17:44.613676 #24027] DEBUG – : Photo Load (0.6ms) SELECT photos.* FROM photos INNER JOIN comments ON comments.photo_id = photos.id ORDER BY photos.id ASC LIMIT 2
D, [2014-08-05T18:17:44.811715 #24027] DEBUG – : Person Load (0.3ms) SELECT people.* FROM people WHERE people.id IN (1)
user1
D, [2014-08-05T18:17:44.845897 #24027] DEBUG – : Person Load (0.6ms) SELECT people.* FROM people WHERE people.id = 1 LIMIT 1
user1
Even though I said includes(:person), and it did cause ActiveRecord to issue the second query to get all the people, when I retrieve the second Photo’s Person ActiveRecord has to query it again. With a production query with many more photos and people it’s very slow.
Is this a bug? Is includes not supported with a query that returns the same object more than once? Is there a way around the N + 1 queries?
Thanks!
