Rails 中的implicit_order_column 和default_scope 有什么区别？

有什么区别：

self.implicit_order_column = 'id'

and

default_scope { order('id ASC') }

self.implicit_order_column允许您使用另一列，然后使用主键作为隐式排序列。这会影响方法的方式，例如.first and .last work:

User.class_eval do 
  self.implicit_order_column = 'created_at'
end

User.first
# => SELECT "users".* FROM "users" ORDER BY "users"."updated_at" ASC LIMIT $1  [["LIMIT", 1]]

User.last
# => SELECT "users".* FROM "users" ORDER BY "users"."updated_at" DESC LIMIT $1  [["LIMIT", 1]]

Setting self.implicit_order_column = 'id'完全没有意义，因为无论如何默认值都是主键列。这implicit_order_column如果您提供明确的命令，当然不会使用。它实际上并没有改变该类产生的任何其他作用域。

default_scope另一方面，将默认范围附加到您从类中生成的任何范围。

irb(main):001:0> User.all
   (0.5ms)  SELECT sqlite_version(*)
  User Load (0.1ms)  SELECT "users".* FROM "users" LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<User id: 1, role: "admin", created_at: "2020-11-08 19:31:31", updated_at: "2020-11-08 19:31:47">]>
irb(main):002:1* User.class_eval do
irb(main):003:1*   default_scope { order(id: :asc) }
irb(main):004:0> end
=> [#<Proc:0x00000000043703a8 (irb):3>]
irb(main):005:0> User.all
  User Load (0.2ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<User id: 1, role: "admin", created_at: "2020-11-08 19:31:31", updated_at: "2020-11-08 19:31:47">]>
irb(main):006:0> 

这里的差异并不是立即显而易见的。但是，如果您考虑到在 SQL 世界中，没有 order 子句的查询将不会按确定的顺序返回记录（取决于其实现），而这里我们实际上是按确定的顺序获取记录。在许多 RDBMS 中，结果很难区分，因为它们可能会按照修改的顺序返回记录（如果它们愿意的话）。

看起来很棒，直到你意识到多么不直观default_scope是什么以及它会导致多少错误。

irb(main):006:0> User.all.order(:created_at)
  User Load (0.2ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC, "users"."created_at" ASC LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<User id: 1, role: "admin", created_at: "2020-11-08 19:31:31", updated_at: "2020-11-08 19:31:47">]>
irb(main):007:0>

或者这个例子：

irb(main):001:1* User.class_eval do
irb(main):002:1*   default_scope { where(admin: true) }
irb(main):003:0> end
=> [#<Proc:0x0000000002bde460 (irb):2>]
irb(main):004:0> User.new
   (0.6ms)  SELECT sqlite_version(*)
=> #<User id: nil, role: "visitor", created_at: nil, updated_at: nil, admin: true>

Yikes! default_scope因此被广泛认为是邪恶的。

See:

• https://github.com/rails/rails/pull/34480 https://github.com/rails/rails/pull/34480
• https://rails-bestpractices.com/posts/2013/06/15/default_scope-is-evil/ https://rails-bestpractices.com/posts/2013/06/15/default_scope-is-evil/
• https://www.reddit.com/r/rails/comments/ha6gqj/default_scope_is_evil/ https://www.reddit.com/r/rails/comments/ha6gqj/default_scope_is_evil/