How to monkey-patch code that gets auto-loaded in

Question

I'm monkey-patching a Rails engine with something like:

SomeClass.class_eval do
  # ...
end

The first time I hit the web site, on development mode at least, it works, but the second time it's like my patch never existed. I presume it's Rails auto-reloading the engine (which is installed in vendor/) and not reloading my code. This is Rails 2.3.

Any ideas how to do it so that my code also gets reloaded?

Answer 1

I just wrote my first monkey-patch, and so needed to come up with a set of conventions around it. Here's what I came up with:

1. Place your extensions under lib/ext/. (Suggested by veteran workmad3 in #rubyonrails IRC room.) In my case, I'm adding a method to the Mail::Message class (from the mail gem, used by ActionMailer), so I created:

   /lib/ext/mail/message.rb

2. Open the class or module and add your code:

   module Mail class Message def to_is_phone? !!(self.to.first =~ /^\+1\d{10}$/) end end end

3. Create an initalizer to load all your monkey-patches. Rails will autoload a file when a constant is referenced, but since you're adding methods to existing classes/modules rather than defining new ones, that won't work, so you have to manually require all your monkey-patches. So I created:

   /config/initializers/monkey_patches.rb

   Which contains:

   require 'ext/mail/message'

Answer 2

It's ugly, but I found that if I put this kind of code at the bottom of environments.rb it always guaranteed correct load-order on startup.

Answer 3

EDIT: This solution only works for Rails 3+ since it's dependent on some functionality in Rails::Railtie. Put this code in an initializer.

This question is quite old, but here's a solution I found:

Rails.configuration.to_prepare do
  SomeClass.class_eval do
    # ...
  end
end

This forces Rails to reload the class on every request in development mode, but only once in production.

Answer 4

Have a look at how this gem handles "decorating" aka monkey patching something in an engine or vice versa:

https://github.com/EPI-USE-Labs/activesupport-decorators

Answer 5

If you place the patch in any .rb file inside /config/initializers, it should work.

Answer 6

Unfortunately, there is no way to hook into the reloading mechanism of Rails 2.x. What you could do, is place your patch somewhere in the app or lib directory. (lib/core_ext is probably the preferred location). Then add the directory to the autoload_paths in your config.

You might also need to open the class, rather than using class_eval.