Rails 5 的基本原理加入 Table 生成的默认值

Question

Rails 5 命令 rails g migration create_foo_bar_join_table 生成以下迁移：

class CreateFooBarJoinTable < ActiveRecord::Migration[5.0]
  def change
    create_join_table :foos, :bars do |t|
      # t.index [:foo_id, :bar_id]
      # t.index [:bar_id, :foo_id]
    end
  end
end

为什么生成器用复合键存根两个（双向）索引？还有为什么他们被注释掉了？我对此感到困惑，找不到任何关于这些建议默认值的明确解释。

以上索引是否比以下索引更高效？

...
    create_join_table :foos, :bars do |t|
      t.index :foo_id
      t.index :bar_id
    end
...

Answer 1

阅读 has_and_belongs_to_many 上的文档时偶然发现了确切的答案：

It’s also a good idea to add indexes to each of those columns to speed up the joins process. However, in MySQL it is advised to add a compound index for both of the columns as MySQL only uses one index per table during the lookup.

https://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_and_belongs_to_many

Rails 5 的基本原理加入 Table 生成的默认值

Rationale of Rails 5 Join Table Generated Defaults

indexing

join

rails-generators

ruby-on-rails-5