contents/articles/simply-on-rails-4-default-data-migrations.html
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title: Simply On Rails - Part 4: Quick and Easy Default Data Migrations
content-type: article
timestamp: 1189854600
tags: rails|ruby|databases
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<p>In the <a href="http://www.h3rald.com/blog/simply-on-rails-3-shared-controller">last post</a> of this series I tried to find a <acronym title="Don't Repeat Yourself"><span class="caps">DRY</span></acronym> solution to deal with tables storing “ancillary” data, i.e. names of user roles, predefined categories, page state names and other similar things.<br />
I personally chose to put this kind of data to make my application more dynamic, although I could have decided to use ENUMs or simply ordinary varchar fields — that would have been easier, but less flexible. For now, I’m sticking with my original choice.</p>
<p>The data in these tables is kind of a prerequisite for the application to run: I must be able to have a status to assign to a user when creating it, and the same applies to roles. Sure, I could spend 20 minutes populating these tables manually, but it would be nice if there was a less tedious way, wouldn’t it?</p>
<p>There is indeed. The inspiration came from a technique described in the book (which I highly recommend) <em>Agile Web Development With Rails</em>, in which the author outlines how it would be possible to use Rails’ fixtures and migrations to load data in the database automatically from <span class="caps">YAML</span> files. <br />
All you have to do is create a migration to load the specified <span class="caps">YAML</span> files and you’re all set.</p>
<p>I wanted to take a little step further, allowing the migration to load data from <em>all <span class="caps">YAML</span> files in a specific directory</em>, automatically.Let’s start creating the <span class="caps">YAML</span> files then and place them all in one directory of the application like <code>/db/migrate/defaults</code>. Here’s the one I used for user roles, for example:</p>
<div class='yaml'><pre><code>visitor:
id: 1
name: Visitor
level: 0
user:
id: 2
name: User
level: 10
contributor:
id: 3
name: Contributor
level: 20
provider:
id: 4
name: Provider
level: 50
operator:
id: 5
name: Operator
level: 100
administrator:
id: 6
name: Administrator
level: 500
webmaster:
id: 7
name: Webmaster
level: 1000</code></pre></div><p>The important thing to remember is to provide a unique string to identify each record, before specifying each fiels. The other files look similar, so I won’t bother listing them here.</p>
<p>And here’s the simple code for the migration:</p>
<div class='ruby'><pre><code>require 'active_record/fixtures'
class LoadDefaults < ActiveRecord::Migration
def self.up
down
models = self.default_models
models.each do |m|
Fixtures.create_fixtures(self.default_directory, m)
end
end
def self.down
models = self.default_models
models.each do |m|
eval("#{m.singularize.capitalize}.delete_all")
end
end
def self.default_directory
File.join(File.dirname(__FILE__), "defaults" )
end
def self.default_models
files, names = Dir.glob("#{self.default_directory}/*.yml"), []
unless files.blank?
files.each { |f| names << File.basename(f, '.yml') }
names
else
[]
end
end
end</code></pre></div><p>Basically the migration will look in a directory named “defaults” for some <span class="caps">YAML</span> files named after a particular database table, and it will attempt to load all the records defined in each one of them. <br />
The <code>down</code> method of the migration <em>deletes all the data in the specified tables</em>, so use with care…</p>
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