Unexpected behavior with ActiveRecord includes - sql

Question

I'm using the AR includes method to execute a LEFT OUTER JOIN between objects User and Building, where a User may or may not have a Building association:
users = User.includes(:building).references(:buildings)
Since I'm using references, any associated Building objects will be eager loaded.
My expectation was that I would then be able to iterate through the list of users, and check whether a user had a building associated with them without triggering additional queries, but I see that in fact whenever I try to access the building property of a user that doesn't have one, AR makes another SQL call to try and retrieve that building (though on subsequent tries it will just return nil).
These queries are obviously redundant as the association would have been loaded during the initial join, and seems to defeat the whole purpose of eager loading with includes/references, as now I'm looking at N times the number of queries equal to the number of empty associations.
users.each do | user |
# This will trigger a new query when building is not present:
# SELECT "buildings".* FROM "buildings" WHERE "buildings"."address" = $1 LIMIT 1 [["address", "123 my street"]]
if user.building
puts 'User has building'
else
puts 'User has no building'
end
end
User class:
class User < ActiveRecord::Base
belongs_to :building, foreign_key: 'residence_id'
end
Is there a way to check the presence of the users' building association without triggering extra queries?
ON RAILS 4.2.0 / POSTGRES
UPDATE:
Thank you #BoraMa for putting together this test. Looks like we're getting different behavior across recent Rails versions:
OUTPUT (RAILS 4.2.0):
User 1 has building
User 2 has building
User 3 has no building
D, [2016-05-26T11:48:38.147316 #11910] DEBUG -- : Building Load (0.2ms) SELECT "buildings".* FROM "buildings" WHERE "buildings"."id" = $1 LIMIT 1 [["id", 123]]
User 4 has no building
OUTPUT (RAILS 4.2.6)
User 1 has building
User 2 has building
User 3 has no building
User 4 has no building
OUTPUT (RAILS 5.0.0)
User 1 has building
User 2 has building
User 3 has no building
User 4 has no building
Take aways:
This issue was limited to "dangling foreign keys (ie the residence_id
column is not nil but there is no corresponding building object)"
(THANKS #FrederickCheung)
The issue has been resolved as of Rails 4.2.6

Answer 1

Sounds like you got bit by a bug in Active Record, that was fixed in rails 4.2.3.
In the case where the column was nil Active Record already knows that it doesn't even need to try loading the associated object. The remaining cases were the ones impacted by this bug

Answer 2

Seems like a typo, please notice building instead of buildings: User.includes(:building).references(:buildings)
That should trigger the big query that uses the format of AS tX_rY for each association and table.

Answer 3

It seems that since rails 4.1 there are potential clashes with how just how implicit #includes should be, see the following open issue.
This code is all untested for syntax, but there would be two approaches I would try:
1/ Make the eager loading implicit
users = User.eager_load(:building).preload(:buildings)
2/ Separate out the two types of users, ones where the building is attached, meaning you don't even try and preload the building, removing the innefficiency.
users = User.includes(:building).where.not(residence_id: nil).references(:buildings)
users.each do | user|
puts "User has building: #{user} #{user.building}"
end
# No additional references needed to be eager-loaded.
users = User.where(residence_id: nil)
users.each do | user |
puts "#{User} has no building."
end