FAQ: How can I convert my Rails app to a Merb app?

So you're heard that Merb is the hot-diggity and you're ready to convert your latest greatest Rails creation to Merb. Below are some basics to get you started.

THIS IS VERY MUCH A WORK IN PROGRESS. Please edit to make it better.

So you're familiar with Rails. It's working great for your app. Things are running well and plenty fast. Do you have a real reason to convert? Many Rails apps are just fine (and will live and die quite happily) being Rails apps their entire life-cycle. It's not an either/or world out there. You may want to use Rails for one project and Merb for another. If you're happy with Rails… it may be time to stop reading.

It could be you've hit the spot where Rails isn't shining as much as it used to…

• perhaps your Rails stack is sweating with millions of hits each hour and you've hear Merb is _fast_
• perhaps you're wanting to port your app just to learn your way around Merb
• perhaps you're constantly fighting with Rails and the “Rails Way”

You've heard Merb is ORM agnostic. This is good news for the Rails conversion. Below are steps to convert your models to Merb.

• copy models/*.rb to your new merb app
• edit your config/init.rb and set your ORM to AR

  use_orm :activerecord 

• edit your config/database.yml to point to your dev, testing, and production databases

Yep, that's it… you're done. You can now fire up merb and hit up your models…

# merb -i

>> Forum.find(:first)
=> #<Forum ...>
>>

• there is no default render assumption in Merb… you must always call render or display if you want anything rendered
• respond_to is replaced with case content_type and provides functionality
• typical params (id, object hash) will be passed to your controller method as parameters (no need to use the params[] hash to retrieve them)

Here is a typical update action in Rails.

  def update
    @forum = Forum.find(params[:id])

    respond_to do |format|
      if @forum.update_attributes(params[:forum])
        flash[:notice] = 'Forum was successfully updated.'
        format.html { redirect_to(@forum) }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @forum.errors, :status => :unprocessable_entity }
      end
    end
  end

Here is the same action in Merb (with AR).

  def update(id, forum)
    provides :html, :xml
    @forum = Forum.find(id)
    if @forum.update_attributes(forum)
       return render(:nothing => 200) if content_type==:xml
       redirect resource(@forum), :message => {:notice => 'Forum was successfully updated.'}
    else
      return render(:xml => ?) if content_type==:xml
      display @forum, :edit
    end
  end

someapp/config/environments/development.rb

rails:

config.after_initialize do
  ActiveMerchant::Billing::Base.mode = :test
end

merb:

Merb::BootLoader::after_app_loads do
  ActiveMerchant::Billing::Base.mode = :test
end