在Rails 3中如何/何时/何处扩展Gem类（通过class_eval和Modules）？

Question

What is the recommended way to extend class behavior, via class_eval and modules (not by inheritance) if I want to extend a class buried in a Gem from a Rails 3 app?

An example is this:

I want to add the ability to create permalinks for tags and categories (through the ActsAsTaggableOn and ActsAsCategory gems).

They have defined Tag and Category models.

I want to basically do this:

Category.class_eval do
  has_friendly_id :title
end

Tag.class_eval do
  has_friendly_id :title
end

Even if there are other ways of adding this functionality that might be specific to the gem, what is the recommended way to add behavior to classes in a Rails 3 application like this?

I have a few other gems I've created that I want to do this to, such as a Configuration model and an Asset model. I would like to be able to add create an app/models/configuration.rb model class to my app, and it would act as if I just did class_eval .

Anyways, how is this supposed to work? I can't find anything that covers this from any of the current Rails 3 blogs/docs/gists.

Answer 1

I do this as follows, first add a file to config/initializers where you can require the files that contain your extensions:

# config/initializers/extensions.rb
require "#{Rails.root}/app/models/category.rb"
require "#{Rails.root}/app/models/tag.rb"

Then you can just re-open the classes and add whatever else you need:

# app/models/category.rb
class Category
  has_friendly_id :title
end

Only downside is that the server has to be restarted for any changes to these files to take effect, not sure if there is a better way that would overcome that.

Answer 2

You can use rails_engine_decorator gem: https://github.com/atd/rails_engine_decorators

Just add in your Gemfile:

gem 'rails_engine_decorator'

And user class_eval in your decorators:

/app/decorators/models/category_decorator.rb
/app/decorators/models/tag_decorator.rb

It works for me. I hope you find it useful!