Agile Programming, Continuation

OK, I said I would continue some other time and if history was any indication, it would have been a long time. But I had the burning urge to say something and my ardent followers were just holding their breath to read it (:-)).

I talked about conventions and ignorance. Here is a case in point (and actually this is a really good example of how to do a multiple selection list in Ruby On Rails).

Here is how to get started. In a windows command prompt window you type:

cd c:\workspaces
rails msexample
cd msexample
ruby script\generate model Movie title:string producer:string
ruby script\generate model Category label:string
ruby script\generate migration CreateCategoriesMovies

Then you edit db\migrate\003_create_categories_movies.rb as follows:

class CreateCategoriesMovies < ActiveRecord::Migration
 def self.up
   create_table :categories_movies, :id => false do |t|
     t.column :category_id, :integer
     t.column :movie_id, :integer
   end
   add_index :categories_movies, [:category_id, :movie_id], :unique => true
 end

 def self.down
   drop_table :categories_movies
 end
end

This will add the has_and_belongs_to_many (habtm) relationship which is describe here. You need to edit the two models also. Here is app\models\category.rb:

class Category < ActiveRecord::Base
 has_and_belongs_to_many :movies
end

Here is app\models\movie.rb:

class Movie < ActiveRecord::Base
 has_and_belongs_to_many :categories
end

We then create the MySQL database using the following commands:

mysqladmin -u root create msexample_development
rake db:migrate

We will then create a set of standard views to be able to edit our list of movies.

ruby script\generate scaffold Movie

We will further customize the form created for this purpose by the scaffold generator by simply adding 2 lines. Here is the app\views\movies\_form.rhtml:

<%= error_messages_for 'movie' %>

<!--[form:movie]-->
<p><label for="movie_title">Title</label><br/>
<%= text_field 'movie', 'title'  %></p>

<p><label for="movie_producer">Producer</label><br/>
<%= text_field 'movie', 'producer'  %></p>

<p><label for="movie_categories">Categories</label><br/>
<%= select('movie', 'category_ids', Category.find(:all).collect {|p| [ p.label, p.id ] }, { :include_blank => true}, {:multiple => true, :size => 3})  %></p>
<!--[eoform:movie]-->

The last thing we need to do is to create the categories scaffold (the rails thing that will allow us to edit categories).

ruby script\generate controller Categories

We then edit app\controllers\categories_controller.rb:

class CategoriesController < ApplicationController
 scaffold :category
end

We are now ready to start the server and navigate to http://localhost:3000/categories to edit and add a few categories and then http://localhost:3000/movies to edit our movies and assign categories to them.

ruby script\server

In my next post I will describe the huge number of conventions that went into making this possible. And it is worthy to note that this only required 60 lines of code (as the rake stats command show below will tell you) of which 49 were created by the rails generation software:

C:\Documents and Settings\mario\workspaces\radrails2\msexample>rake stats
(in C:/Documents and Settings/mario/workspaces/radrails2/msexample)
+----------------------+-------+-------+---------+---------+-----+-------+
| Name                 | Lines |   LOC | Classes | Methods | M/C | LOC/M |
+----------------------+-------+-------+---------+---------+-----+-------+
| Controllers          |    61 |    48 |       3 |       8 |   2 |     4 |
| Helpers              |     7 |     6 |       0 |       0 |   0 |     0 |
| Models               |     6 |     6 |       2 |       0 |   0 |     0 |
| Libraries            |     0 |     0 |       0 |       0 |   0 |     0 |
| Components           |     0 |     0 |       0 |       0 |   0 |     0 |
| Integration tests    |     0 |     0 |       0 |       0 |   0 |     0 |
| Functional tests     |   110 |    79 |       4 |      13 |   3 |     4 |
| Unit tests           |    20 |    14 |       2 |       2 |   1 |     5 |
+----------------------+-------+-------+---------+---------+-----+-------+
| Total                |   204 |   153 |      11 |      23 |   2 |     4 |
+----------------------+-------+-------+---------+---------+-----+-------+
  Code LOC: 60     Test LOC: 93     Code to Test Ratio: 1:1.6