From 3204575bfcd1f12db5945c8959073d40915cfdfe Mon Sep 17 00:00:00 2001
From: José Mota <josemota.net@gmail.com>
Date: Fri, 6 Apr 2012 19:40:37 +0100
Subject: Import all posts.

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+---
+layout: post
+title: Relationships in Rails 2.3
+tags:
+- Development
+- rails
+status: publish
+type: post
+published: true
+meta:
+  _edit_last: '1'
+---
+Ruby on Rails is becoming quite a piece of software. I've been learning how to work with its latest version to date, 2.3, for a series of workshops on web development. It has surprised me how easy it is now to accomplish a simple task such as multi-model form processing. Let me show what I've been doing lately.
+
+<!--more-->
+<h3>Case study: a school</h3>
+The example I am going to show my attendees resembles a typical school situation. In a nutsheel:
+<ul>
+	<li>Students have Subjects.</li>
+	<li>Subjects have Students.</li>
+	<li>Students have Grades to Subjects.</li>
+</ul>
+[caption id="attachment_78" align="alignnone" width="442" caption="Rails relationships — student"]<a href="/wp-content/uploads/2009/07/rails-relationships-student.png"><img class="size-full wp-image-78" title="Rails relationships — student" src="/wp-content/uploads/2009/07/rails-relationships-student.png" alt="Rails relationships — student" width="442" height="98" /></a>[/caption]
+<h3>The Models</h3>
+<h4>Student</h4>
+<pre class='brush:rails'>class Student < ActiveRecord::Base
+   has_many :grades
+   has_many :subjects, :through => :grades, :uniq => true
+   accepts_nested_attributes_for :grades, :allow_destroy => true
+end</pre>
+<h4>Subject</h4>
+<pre class='brush:rails'>class Subject < ActiveRecord::Base
+  validates_uniqueness_of :shortname
+  has_many :grades
+  has_many :students, :through => :grades, :uniq => true
+end
+</pre>
+<h4>Grade</h4>
+<pre class='brush:rails'>class Grade < ActiveRecord::Base
+  belongs_to :subject
+  belongs_to :student
+  validates_presence_of :value
+end
+</pre>
+<h3>And the magic trick!</h3>
+Through <em>accepts_nested_attributes_for</em>, the student's <em>form_for</em> can now accept nested fields through <em>fields_for</em> for setting Grades' data.
+
+Everytime a student is created, subjects are not directly associated to him. You need to create a form in which you allow to assign a student the subjects you want.
+
+[image missing]
+
+This tells you that a student can have an array of subject id's. You even get to see what subjects are already assigned and dissociate them just like that! When the associations are done, a new Grade record is created to associate the first two modules. Unfortunately, when the associations are dismantled, the Grade association is gone too.
+
+If you know Railscasts, the <a href="http://railscasts.com/episodes?search=complex+forms">Complex Forms series</a> explain this task for Rails &lt; 2.3. But now it has become easier and easier to associate models and worrying less about your structure.
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