ruby-on-rails - 了解“后续”mixin 模块的 `include` 语句行为

Question

我试图了解include在多个 Mixin 中使用时的语句行为。也就是说，我有这些陈述：

class Article < ActiveRecord::Base
  include DoubleIncludedModule
  include AModule

  # ...
end

module AModule
  include DoubleIncludedModule

  # ...
end

module DoubleIncludedModule
  # ...
end

DoubleIncludedModule课程中会包含多少次Article？也就是说，由于DoubleIncludedModule（首先在Article类中，然后在类本身中AModule包含Article）的“后续”包含，将是 Ruby 自动处理的“双重包含”问题，还是会DoubleIncludedModule（错误地）包含两次？

当然，我只想包含该DoubleIncludedModule模块一次。我怎样才能以正确的方式做到这一点（也许通过使用一些 Ruby on Rails 方法）？

Answer 1

我会用一个例子来回答：

module A
  def self.included base
    puts "A included in #{base}"
  end
end

module B
  include A

  def self.included base
    puts "B included in #{base}"
  end
end

class C
  include A
  include B
end

p C.ancestors

印刷

A included in B
A included in C
B included in C
[C, B, A, Object, Kernel, BasicObject]

As we can see, A is included only once into C. Though technically it was included twice since it was included into B which was also included into C. Does this matter? Probably not. Each still only occurs once in the ancestor chain, and any methods would've been overridden with equivalent implementations—i.e., essentially a no-op. Unless you're doing something exotic with the included hook, you're unlikely to notice anything.