Join Rails 5.2中作用域中的连接与Rails 5.1有何不同？

在我将rails从5.1升级到5.2之后，我开始出现以下错误：

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 `[]'

导致错误的代码如下所示：

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

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

我创建了一个包含完整代码的程序，它有助于重现bug。下载

issue_with_joins_in_scopes_in_rails_5_3.rb

到您的电脑，并使用

ruby issue_with_joins_in_scopes_in_rails_5_3.rb

您可以查看更多详细信息

Rails 5.2和5.1中的连接有什么区别，导致代码

Post.with_last_comment

在Rails 5.2中出错而失败

---

如何更改帖子模型中的

last\u comment

关联和

与\u last\u comment

范围，使其在Rails 5.2中工作？

发布关于如何为帖子集合预加载last comment的部分解决方案

首先，您需要有以下

last_comment

关联：

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

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

此解决方案的问题在于，当我使用

联接时，它会忽略关联范围并生成以下SQL查询：

Post.joins(:last_comment)

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

我可以通过以下方式更改原始的
last_comment
关联来解决此问题：

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

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

关于Rails 5.2中的连接与Rails 5.1中的连接如何不同的问题仍然悬而未决