Getting started with Merb and Sequel Part 1

Merb is a new super fast, multi threaded, Ruby based MVC Framework based on Mongrel and Embedded Ruby. Developer of merb is Ezra Zygmuntowicz of engine yard fame. The latest release of merb is based on rack which is common interface between various web servers and ruby frameworks. So now merb is no longer bound to mongrel only.

Merb does not force developers to stick to one thing unlike Rails. The merb allows developers to choose from three different ORM's i.e. Active Record, Data mapper and Sequel. Merb is also Multi threaded as long as you don't use Active Record as ORM, that means you dont need to run pack of mongrels like Rails.

Lets built a small web application to check out merb. In my example i ll be using Sequel ORM with merb. so go ahead and install merb and Sequel. Type this on command line:

% sudo gem install merb --include-dependencies

after that

% sudo gem install merb_sequel

Now lets create application directory structure:

% merb time_management

As you can see we are going to make little time management application for this tutorial. First of all we need to tell merb that we intend to use Sequel in this application. Open file named dependencies.rb in config folder below application root, uncomment this line:

use_orm :sequel

Now simply run merb command from your application root. This will create a file named database.sample.yml in config folder. Change parameters in this file and save it as database.yml in same config folder. The database.yml syntax is similar to rails database.yml file. Here is how database.yml file looks like:

---
# This is a sample database file for the Sequel ORM
:development: &defaults
:adapter: mysql
:database: ts_development
:username: root
:password:
:host: localhost
:socket: /tmp/mysql.sock

:test:
<<: *defaults :database: ts_test

:production:
<<: *defaults :database: ts_production

Now we have database setup lets create a model now. The merb has generators to create models, controllers and specs just like rails.

% ./script/generate model user

this will create bunch of files like model named user.rb in app/models folder. Among these files it also creates migration named like 001_add_model_users.rb in schema/migrations folder. Now lets write some code in this migration to create users table. The Sequel migration syntax is different from Active Record. Look at documentation for more details. Here is how migration would look like after editing:

class AddModelUsers < Sequel::Migration
def up create_table :users do |u|
primary_key :id
varchar :first_name, :size => 100
varchar :last_name, :size => 100
varchar :user_name, :null => false
varchar :password, :null => false
end
end

def down
execute "DROP TABLE users"
end

end

Now that table is created lets create a controller, leave model empty for now later on some business logic can go into models.

% ./script/generate controller main

This will create file named main.rb in app/controllers folder . Notice there is already file name application.rb in controllers folder. All controllers inherit from this Application class so common controller code can go in this Application class. Now lets write login and logout functionality in main.rb file.

class Main < Application

def index
render
end

def login
@error =false
uname = params.fetch(:uname)
pass = params.fetch(:pass)
user = User.filter(:user_name => uname, :password => pass)
unless @user.blank?
session[:user_id] = @user.first.id
redirect "/main/"
else
@error = true
end
end
if session[:user_id] != nil
@logged_in = true
@user = User.filter(:id => session[:user_id]).first
else
@logged_in = false
end
render
end

def logout
session[:user_id] = nil
redirect "/main/"
end
end

There are two things to notice here for rails programmers one is the way you fetch values from params that are passed to each action. We call fetch method on params object to get variables that are passed with the request for that action. Other is Sequel syntax of model which uses filter instead of find. For more detailed syntax take a look at documentation.
Another thing to look at is explicit call to render function. Which renders erb template located in app/views folder e.g. index.html.erb for index action. The templates are pure embedded ruby . The creators of merb claims that it is drop in replacement of rails Action pack. True there is not much difference. Lets change a default route to point to index action of main controller. Change config/router.rb to add this line:

r.match("/").to(:controller => "main", :action => "index")

above this line

r.default_routes

Start our sample application by running merb command in our application root:

% merb

If all goes well you should be able to access your sample application at http://127.0.0.1:4000/ .

Lets add few more things to our application in next part ....