Better callback deprecation warnings for Rails 5.0

rails_5_1_callback_halting_monitor.rb

# A monkey-patch to make detecting deprecated
# callbacks easier, because a stack trace is
# not the greatest when it comes to callbacks.
#
#
# Original code:
# https://github.com/rails/rails/blob/c4d3e202e10ae627b3b9c34498afb45450652421/activesupport/lib/active_support/callbacks.rb#L766-L788
require "active_support/callbacks"

module ActiveSupport
  module Callbacks
    module ClassMethods
      def deprecated_false_terminator # :nodoc:
        Proc.new do |target, result_lambda|
          terminate = true
          catch(:abort) do
            result = result_lambda.call if result_lambda.is_a?(Proc)
            if Callbacks.halt_and_display_warning_on_return_false && result == false
              # the scope that `result_lambda` is created in contains the filter
              # that we need to identify the callback with, so let's pull it
              # out of the binding.
              filter = result_lambda.binding.local_variable_get(:filter) rescue nil
              display_deprecation_warning_for_false_terminator(target, filter)
            else
              terminate = false
            end
          end
          terminate
        end
      end

      def display_deprecation_warning_for_false_terminator(target=nil, filter=nil)
        ActiveSupport::Deprecation.warn(<<~MSG)
          Returned `false` from callback in `#{target.class.name}##{filter}`!

          Returning `false` in Active Record and Active Model callbacks will not implicitly halt a callback chain in Rails 5.1.
          To explicitly halt the callback chain, please use `throw :abort` instead.
        MSG
      end
    end
  end
end

Use the patch like this:

# config/application.rb

# IMPORTANT: the patch needs to be added before booting Rails,
# since it needs to take effect before ActiveRecord loads.
require_relative "../extensions/rails_patches/rails_5_1_callback_halting_monitor"

require_relative "boot"
require "rails/all"

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(*Rails.groups)

module MyApp
  class Application < Rails::Application
    ...
  end
end