Prioritized ActiveSupport Callbacks

Sometimes, when you build big system with a massive number of plugins, you can realize that you need to prioritize callbacks in your models or controllers to have full control which callbacks are executed first, which are executed last despite of the order in which they were initially added.

WARNING! Do not use this gem unless you know that's the only way.

Add the following line to Gemfile:

gem 'prioritized_callbacks'

It is checked against all Rails versions >= 5.0 and Ruby >= 2.7

require 'active_support'
require 'prioritized_callbacks'

class MyClass
  include ActiveSupport::Callbacks
  
  define_callbacks :action, order: [:first, :default, :last]

  set_callback :action, :before do
    puts "I'm called second before action"
  end
  
  set_callback :action, :before, priority: :first do
    puts "I'm called first before action"
  end

  set_callback :action, :before, priority: :last do
    puts "I'm called third before action"
  end
end

MyClass.new.run_callbacks :action do
  puts "Action performed"
end

Will output:

I'm called first before action
I'm called second before action
I'm called third before action
Action performed

Here are two parts to setup:

1. Definition of callbacks priority names and its order:

define_callbacks :action, order: [:first, :default, :last]

When you define callbacks, you specify the order of callback groups. You can give groups any name, but there is a special name :default which is used for any callbacks whose priority is not specified.

If you do not specify :order, it gets default Rails behaviour and is implicitly set to [:default]

2. Assigning priority name to callback itself:

set_callback :action, :before, priority: :first do ... end

When you add new callback, you can specify its priority by using name you used in callback definition (see above). If you don't specify :priority, :default is assumed.

Callbacks with the same priority are executed in the order how they were added.

class Record < ActiveRecord::Base
  
  set_save_order :first, :default, :last
  
  before_save priority: :last do
    # Do something last
  end
  
  before_save :do_first, priority: :first
  
  before_save :do_default
end

1. To set order of save callbacks in ActiveRecord, you should use:

set_save_order ..., :default, ...

Or in common form:

set_callbacks_order :save, [..., :default, ...]

The latter can be used outside of ActiveRecord, in ActionController or ActiveModel, where ActiveSupport::Callbacks are included.

To append the order in any inherited class:

append_save_order :my_order, before: :default

You can specify :before or :after to specify a place where new order item should be inserted.

2. To set priority for specific callback, you should add :priority option to before_save or after_save.