Parts

Parts in merb are just like small controllers that you can use to perform logic. One very good usage for them is when you want add a “widget” to a view, that has its own logic.

We'll look at an example on how to make a part that, displays the news titles.

Merb parts don't come with merb-stack, so you need to get them separately

sudo gem install merb-parts

Generate a part

$ merb-gen part news

[ADDED]  app/parts/news_part.rb
[ADDED]  app/parts/views/news_part/index.html.erb
[ADDED]  app/helpers/news_part_helper.rb

If getting an error

FATAL: The gem merb-action-args (= 0.9.10, runtime), [] was not found

edit your dependencies.rb file an change the version on the top to the (latest) version of merb you have installed.

You will need to add merb_parts as a dependency: In your dependencies.rb file add

dependency "merb-parts", merb_gems_version

Note that currently merb-parts is not tracking merb_gems_version, so may need a separate version

Lets assume we have some News model (merb-gen model news to get one) We want to make our part display the news titles in a pane, that is reusable on other pages.

First, lets get the news form the db in the PartController. Edit app/parts/news_part.rb to look something like this:

class NewsPart < Merb::PartController
  def index
    @news = News.all
    render
  end
end

Nice, we have the data now.

Lets touch the view to show it. Edit app/parts/views/news_part/index.html.erb and make it look similar to this (for example):

<h2>News</h2>
<ul>
  <% @news.each do |news| %>
    <li><%= news.title %></li>
  <% end %>
</ul>

Nice! ¹⁾ Now we have a view for our part action. Lets use it

Using the part is very simple - in any of your views call

  <%= part NewsPart => :index %>

If you have used the part on the controller, responding to '/', go to http://localhost:4000/ and see the titles of the news.

Remember to actually add some news to your database so there is something you can see. :)

Now, let's write a spec

In a BDD/TDD manner - this should have been the first section, but for this tutorial it's fine.

Following the idea from Yehuda to test the whole request instead of various components let's test our '/' route (the one that is displaying the news titles).

Create a file index_spec.rb under spec/request and put this code in it.

require File.join( File.dirname(__FILE__), '..', "spec_helper" )
 
describe '/' do
  before :each do
    # Generate 3 news to help us test
    1.upto(3) do |i|
      News.new(:title => "news#{i}", :content => "this is news #{i}").save
    end
 
  end
 
  it "should have news titles listed" do
    @response = request('/') # Make a request to '/'.
    @response.should be_successful # Request is ok
    @response.should have_xpath('//ul/li[3]') # We have at least 3 news titles
  end
 
end

On a side - to make your test auto_migrate! our database when our tests are run add the following line to spec/spec_helper.rb

Spec::Runner.configure do |config|
  config.include(Merb::Test::ViewHelper|>)
  config.include(Merb::Test::RouteHelper|>)
  config.include(Merb::Test::ControllerHelper|>)
  DataMapper.auto_migrate! # (Re)Create our database
end

When calling the part controller form the view you can pass additional parameters, that get mixed in the params hash in the controller.

  <%= part NewsPart => :index, :max => 3 %>

The :index action of the NewsPart should be updated like

  def index
    limit = params[:max] || 2
    @news = News.all(:limit => limit)
    display @news
  end

If you are using merb-action-args the former can be rewritten as

  def index(max = 2)
    @news = News.all(:limit => max)
    display @news
  end