Ruby 2+中如何结合ActiveSupport::Concern使用prepend？

Why the Original Concern Approach Fails #

First, let's break down exactly why this happens. When you use ActiveSupport::Concern with an included block, that block only executes when the module is included into a class—not when it’s prepended.

Your original non-Concern Baz module worked because you defined the foo method directly on the module itself. When you prepend Baz to class A, Ruby places Baz at the front of A's method lookup chain, so Baz#foo gets called first, and super correctly delegates to A#foo.

But with your BazConcern using included, the foo method is injected directly into class A (not stored on the module). When you prepend BazConcern, the module itself has no foo method, so Ruby skips it entirely and calls A#foo directly.

Fixes for Using prepend with ActiveSupport::Concern #

There are two clean, idiomatic ways to make this work in Ruby 2+, depending on whether you need Concern-specific features like class methods or hooks.

Option 1: Define Instance Methods Directly on the Concern #

This is the simplest approach, mirroring your original non-Concern module. Just define the instance method directly on the Concern instead of wrapping it in an included block:

require 'active_support/concern'

module BazConcern
  extend ActiveSupport::Concern

  # Define the instance method directly on the module
  def foo
    super # Calls A#foo as expected
    :baz   # Returns the value per your original logic
  end
end

class A
  prepend BazConcern

  def foo
    :bar
  end
end

A.new.foo # => :baz (works perfectly!)

This works because prepend places BazConcern at the top of A's method lookup chain, so BazConcern#foo runs first, and super correctly references A#foo.

Option 2: Use prepended Hook for Class-Level Logic #

If you need to run class-level code when the module is prepended (like adding callbacks, associations, or class methods), use ActiveSupport::Concern's prepended hook instead of (or alongside) included. Keep instance methods directly on the module:

require 'active_support/concern'

module BazConcern
  extend ActiveSupport::Concern

  # Runs when the module is prepended to a class
  prepended do
    # Example class-level logic: add a save callback
    before_save :log_save_event
  end

  # Instance method still defined directly on the module
  def foo
    super
    :baz
  end

  # Class methods (works with both include and prepend)
  module ClassMethods
    def baz_class_method
      :baz_class_value
    end
  end

  private

  def log_save_event
    puts "Saving #{self.class} instance..."
  end
end

class A
  prepend BazConcern

  def foo
    :bar
  end
end

A.new.foo # => :baz
A.baz_class_method # => :baz_class_value

The prepended hook runs in the target class's context, so you can add any class-level configuration you need there, while the instance method remains on the module to be picked up by the prepend operation.

Key Takeaways #

• included hooks only trigger on include, not prepend
• To use prepend with ActiveSupport::Concern, define instance methods directly on the module
• Use the prepended hook for class-level logic when required

内容的提问来源于stack exchange，提问作者Matrix