https://api.rubyonrails.org/classes/ActiveSupport/Concern.html：
一个典型的模块看起来像这样：

module M
  def self.included(base)
    base.extend ClassMethods
    base.class_eval do
      scope :disabled, -> { where(disabled: true) }
    end
  end

  module ClassMethods
    ...
  end
end

通过使用ActiveSupport::Concern，上面的模块可以写成：

require 'active_support/concern'

module M
  extend ActiveSupport::Concern

  included do
    scope :disabled, -> { where(disabled: true) }
  end

  class_methods do
    ...
  end
end

此外，它优雅地处理模块依赖关系。给定一个Foo模块和一个依赖于前者的Bar模块，我们通常会写如下：

module Foo
  def self.included(base)
    base.class_eval do
      def self.method_injected_by_foo
        ...
      end
    end
  end
end

module Bar
  def self.included(base)
    base.method_injected_by_foo
  end
end

class Host
  include Foo # We need to include this dependency for Bar
  include Bar # Bar is the module that Host really needs
end

但是Host为什么要关心Bar的依赖关系，即Foo呢？我们可以尝试直接将Foo隐藏在Bar中：

module Bar
  include Foo
  def self.included(base)
    base.method_injected_by_foo
  end
end

class Host
  include Bar
end

不幸的是，这是行不通的，因为当包含Foo时，它的基础是Bar模块，而不是Host类。使用ActiveSupport::Concern，模块依赖关系可以正确解决：

require 'active_support/concern'

module Foo
  extend ActiveSupport::Concern
  included do
    def self.method_injected_by_foo
      ...
    end
  end
end

module Bar
  extend ActiveSupport::Concern
  include Foo

  included do
    self.method_injected_by_foo
  end
end

class Host
  include Bar # It works, now Bar takes care of its dependencies
end

使用ActiveSupport：：关注常规mixin的一个原因是它延迟加载包含。因此，如果您引用模型、控制器等中的代码，它将解析。然而，当加载mixin时，您可能会得到未定义的错误。