initializer for determining where callbacks were registered

callback_registration_tracing.rb

# frozen_string_literal: true

module CallbackRegistrationTracing
  module Capture
    def normalize_callback_params(*args)
      super.tap do |_, _, options|
        options[:_caller] = caller(3)
      end
    end
  end

  module Keep
    def initialize(name, filter, kind, options, chain_config)
      @caller = options.delete(:_caller)
      super
    end
  end

  ActiveRecord::Base.extend Capture
  ActiveSupport::Callbacks::Callback.prepend Keep
end if Rails.env.development? && ENV['CALLBACK_REGISTRATION_TRACING']

This depends very much on undocumented Rails internals which can change without warning.

This version known to work with Rails 6.0.