How are joins in scopes in Rails 5.2 different from rails 5.1?

Question

After I upgraded rails from 5.1 to 5.2 I started getting the following error:

NoMethodError: undefined method `expr' for nil:NilClass
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency/join_association.rb:47:in `block in join_constraints'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency/join_association.rb:33:in `reverse_each'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency/join_association.rb:33:in `join_constraints'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency.rb:167:in `make_constraints'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency.rb:177:in `make_join_constraints'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency.rb:104:in `block in join_constraints'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency.rb:103:in `each'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency.rb:103:in `flat_map'
  from /gems_path/activerecord-5.2.0/lib/active_record/associations/join_dependency.rb:103:in `join_constraints'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation/query_methods.rb:1026:in `build_join_query'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation/query_methods.rb:1008:in `build_joins'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation/query_methods.rb:928:in `build_arel'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation/query_methods.rb:903:in `arel'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation.rb:554:in `block in exec_queries'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation.rb:578:in `skip_query_cache_if_necessary'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation.rb:542:in `exec_queries'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation.rb:414:in `load'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation.rb:200:in `records'
  from /gems_path/activerecord-5.2.0/lib/active_record/relation/delegation.rb:41:in `[]'

The code which causes the error looks the following way:

class Post
  has_many :comments

  has_one :last_comment, -> {
    joins("LEFT JOIN posts on posts.id = comments.post_id")
    .where("
      comments.id = (
        SELECT MAX(comments.id) FROM comments
        WHERE comments.post_id = posts.id
      )"
    )
  }, class_name: "Comment"

  scope :with_last_comment, -> { joins(:last_comment) }
end

I created this gist which contains full code which helps to reproduce the bug. Download issue_with_joins_in_scopes_in_rails_5_3.rb to your PC and run it with

ruby issue_with_joins_in_scopes_in_rails_5_3.rb

You can look at this Github issue for more details

What is the difference between joins in Rails 5.2 and 5.1 which causes the code Post.with_last_comment to fail with error in Rails 5.2?

How can I change last_comment association and with_last_comment scope in the Post model so it will work in Rails 5.2?

Answer 1

Posting partial solution on how to preload last comment for a collection of posts

First you will need to have the following last_comment association:

class Post < ApplicationRecord
  has_many :comments, dependent: :destroy

  has_one :last_comment, -> { order(id: :desc) }, class_name: "Comment"
end

It works with preload and includes in Rails 5.2 and generates the following SQL queries:

Post.includes(:last_comment)

Post Load (0.2ms)  SELECT  "posts".* FROM "posts" LIMIT ?  [["LIMIT", 11]]
Comment Load (0.2ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" = ? ORDER BY "comments"."id" DESC  [["post_id", 1]]

The problem with this solution is that when I use joins it ignores association scope and generates the following SQL query:

Post.joins(:last_comment)

Post Load (0.2ms)  SELECT  "posts".* FROM "posts" INNER JOIN "comments" ON "comments"."post_id" = "posts"."id" LIMIT ?  [["LIMIT", 11]]

I was able to solve it changing the original last_comment association the following way:

class Post < ApplicationRecord
  has_many :comments, dependent: :destroy

  has_one :last_comment, -> {
    joins(:post)                               # <--- Note this change
    .where("
      comments.id = (
        SELECT MAX(comments.id) FROM comments
        WHERE comments.post_id = posts.id
      )"
    )
  }, class_name: "Comment"

  scope :with_last_comment, -> { joins(:last_comment) }
end

And now Post.with_last_comment generates the following SQL:

Post Load (0.3ms)  SELECT  "posts".* FROM "posts" INNER JOIN "comments" ON "comments"."post_id" = "posts"."id" INNER JOIN "posts" "posts_comments" ON "posts_comments"."id" = "comments"."post_id" AND (
      comments.id = (
        SELECT MAX(comments.id) FROM comments
        WHERE comments.post_id = posts.id
      )) LIMIT ?  [["LIMIT", 11]]

The question on how joins in Rails 5.2 are different from Rails 5.1 is still open