Rails mountable engine: how should apps set configuration variables?

Figured out an even better solution that also allows you to set default values (incase the app using the engine doesn't specify a config)...

1. Create config variables in your app's /config/initializers/blog.rb like this:

    Blog.setup do |config|
        config.site_name = "My Site Name"
    end

2. In your engine's /lib/blog/engine.rb set default values like this:

    module Blog
    
         self.mattr_accessor :site_name
         self.site_name = "Site Name"
         # add default values of more config vars here
    
         # this function maps the vars from your app into your engine
         def self.setup(&block)
            yield self
         end
    
    end

3. Now you can simply access the config variables in your engine like this:

Blog.site_name

Much cleaner.

After a lot of testing and looking into existing gems, here is what works in Rails 4:

Considering your engine's name is Blog:

In your engine's /lib/blog/engine.rb put in this:

module Blog

    def self.setup(&block)
        @config ||= Blog::Engine::Configuration.new

        yield @config if block

        @config
    end

    def self.config
        Rails.application.config
    end

end

In your app, create a file called /config/initalizers/blog.rb and set config vars like this:

Blog.setup do |config|
    config.testingvar = "asdfasdf"
end

Then you can access these config variables ANYWHERE in your engine like this:

Blog.config.testingvar

Hope this helps someone. There is very little documentation on this right now so it was all trial and error.