Sample Application Using Edge Merb and DataMapper

Okay, now that you have edge Merb and DataMapper installed, let's create our first app. Pick a directory where you want to put your app, and try the following:

$ merb-gen app sample

You should see "Generating with app generator:" followed by a list of the new directories and files Merb added for you. Change into that directory:

cd sample

and edit the config/init.rb file. Uncomment the DataMapper ORM line:

use_orm :datamapper

and also add a dependencies on merb_helpers and merb-assets:

# ==== Dependencies

# These are some examples of how you might specify dependencies.
# Dependency loading is delayed to a later Merb app
# boot stage, but it may be important when
# another part of your configuration relies on libraries specified
# here.
#
dependencies "merb_helpers", "merb-assets"

Finally, if you want to add query logging from DataMapper, add this to the 'after_app_loads' block:

Merb::BootLoader.after_app_loads do

  DataObjects::Mysql.logger = DataObjects::Logger.new('log/dm.log', 0)

end

Now, let's set up our databases:

$ rake dm:db:database_yaml

Rename (or move) the generated config/database.yml.sample file to config/database.yml. Then go ahead and enter the correct values for the setup you want. Here's how I have mine:

development: &defaults
# These are the settings for repository :default
adapter:  mysql
database: sample_development
username: root
password:
host:     localhost

test:
<<:       *defaults        
database: sample_test            

production:        
<<:       *defaults         
database: sample_production      

Next, let's create a resource:

merb-gen resource Post title:string body:text created_at:date_time

(NOTE: Run merb-gen with no arguments to see a complete list of generators.)

Make sure that you use 'date_time' and not 'datetime' as you would with ActiveRecord. Otherwise, the created_at field will be assigned the type 'Datetime' (which is wrong), and not the correct 'DateTime'. One other potential gotcha - DataMapper strings will generate a varchar(50) field in MySQL, not varchar(255) like in ActiveRecord.

The merb-gen command will generate a Post model at app/models/post.rb, among other files. Take a look at the file now:

class Post
include DataMapper::Resource

property :title, String
property :body, Text
property :created_at, DateTime

end

You will need to add a property (I put it above the other properties):

property :id, Integer, :serial => true

because DataMapper requires you to explicitly declare the primary key. Next, go ahead and run the migration:

rake dm:db:automigrate

Note that the dm:db:automigrate task is destructive. If you create any posts, then run the task again, the table will be dropped and recreated, causing you to lose your data.

You will also need to edit config/router.rb, because Merb doesn't add the Post resource to the routes automatically:

Merb::Router.prepare do |r|
# RESTful routes
r.resources :posts
end

Now let's take a look at the views (the posts controller at app/controllers/posts.rb should have everything you need from the merb-gen resource command). I modified my app/views/posts/new.html.erb file to look like this:

<h1>New Post</h1>
<%= error_messages_for :post %>

<% form_for(:post, :action => url(:posts) do %>
<%= partial('form', :button_text => 'Create') %>
<% end %>

<%= link_to 'Back', url(:posts) %>

I moved the form elements into a form partial at app/views/posts/_form.html.erb:

<p>
<label for="post_title">Title: </label><br/>
<%= text_control :title %>
</p>
<p>
<label for="post_body">Body: </label><br/>
<%= text_area_control :body %>
</p>
<p>
<%= submit_button button_text %>
</p>

You can probably guess what the edit view looks like.

Let's fire up merb and take a look in the browser:

$ merb

(NOTE: enter 'merb --help' to see a complete list of options to the 'merb' command).

Now open up your favorite browser and navigate to 'http://localhost:4000/posts'. You should see something very like this:

Now go ahead and create your first post!