Although migrations are a very cool feature of Ruby on Rails, , they are not covered in any of the basic books on RoR i have encountered so far (Agile Web Development with Rails, , Ruby for Rails Programmers).

Update: Check out my recent post: Ruby on Rails Migrations: Reloaded for an update.

Both these books are using an ‘in medias res’ style approach - they guide the reader through the essential features of Rails by building a web app from scratch. , The models in the examples are creaed in SQL rather than with migrations. , Let’s examine the difference on a simple example, , taken from AWDwR. , (Further I am assuming that you have generated a rails application, , a development database for the application and the DB connection settings (database.yaml) are correct.)

The classic way: SQL DDL

Create the sql file, , create.sql:

drop table if exists products;
create table products (
id           int            not null auto_increment, ,
title        varchar(100)   not null, ,
description  text           not null, ,
image_url    varchar(200)   not null, ,
price        decimal(10, ,2)  not null, ,
primary key (id)
);

After this, , you can create the table with:

mysql name_of_your_DB < create.sql

You are now ready to generate your model.

Doing the same with migrations

In your rails app directory, , issue the following command:

ruby script/generate migration ProductMigration

then open the file db/migrate/001productmigration.rb and edit it. , To achieve the same result as in the SQL example, , the file should look like this:

class ProductMigration < ActiveRecord::Migration

def self.up create_table :products do |table|

table.column :title, ,
:string, ,
:limit => 100, ,
:null => false

table.column :description, ,
:text, ,
:null => false

table.column :image_url, ,
:string, ,
:limit => 200, ,
:null => false

table.column :price, ,
:float, ,
:null => false
end
end

def self.down
drop_table :products
end
end

Run the migration wit the following command:

rake migrate [VERSION=version_number]

And you achieved the same result as with the first method!

That’s very nice, , but…

Well, , if the only purpose of migrations would be solely the possibility to write Ruby code instead of SQL, , even this would be enough for me to go for them. , However, , i have to admit that this alone would be a rather feeble argument. , The good news is that it is not! There is much more to migrations than writing Ruby code:

• Migrations are DB agnostic - The ‘write once, , use everywhere’ principle really works here!
• You don’t have to think about obscure SQL specific things anymore - let Rails handle them for you! (OK there are some really complicated things, , but fortunately they are adressed by some great books like Rails Recipes, , code snippets like Migrate Plus, , and I believe that by the Rails team, , too.)
• You can change the database as much as you want, , and the data you have already there is not affected.
• You get very effective versioning: track changes, , concurrent versions, , upgrade/downgrade your schemas easily!
• You can generate DB schemas from migrations.
• And possibly much much more… , I am a newbie too!

In my oppinion, , judging based on the Rails mailing list discussions, , migrations are accepted more and more as the definitve way of creating, , maintaining, , versioning your DB models - so everybody considering serious Rails development should give them a look!

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This entry was posted on Wednesday, May 3rd, 2006 at 9:37 am and is filed under Rails, Tutorial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.