Marking Your Database Versions

Camping apps are little independent apps, often a single file, which creates the database tables and manages them without hassle, right? So what if you put out a new version of your Camping app? How do you get new tables into the database? Or, what if you need to change a field or something?

In your Models, you can have versioned classes which describe how to upgrade or downgrade the database. Here's the most common scenario:

 #!ruby
 module Blog::Models
   class CreatePostsAndUsers < V 1.0

     def self.up
       create_table :blog_posts, :force => true do |t|
         t.column :user_id,  :integer, :null => false
         t.column :title,    :string,  :limit => 255
         t.column :body,     :text
       end
       create_table :blog_users, :force => true do |t|
         t.column :username, :string
         t.column :password, :string
       end
     end

     def self.down
       drop_table :blog_posts
       drop_table :blog_users
     end

   end
 end

Apps start out with a version of 0.0. The above describes an upgrade to version 1.0, which creates the blog_users and blog_posts table. (In Camping, tables must always start with the name of your app!) I also describe a downgrade, which involves dropping both tables so I can start from scratch, if I ever want to.

Be Sure to Change Your create Method

To kick this all into gear, the create_schema method needs to be called. Put this in your create method so that the tables will get built when you start the app.

 #!ruby
 def Blog.create
   Blog::Models.create_schema
 end

Adding the Next Version

When I release 1.1 of the blog, I should definitely add comments.

 #!ruby
 module Blog::Models
   class CreateComments < V 1.1

     def self.up
       create_table :blog_comments, :force => true do |t|
         t.column :post_id,  :integer, :null => false
         t.column :username, :string
         t.column :body,     :text
       end
     end

     def self.down
       drop_table :blog_comments
     end

   end
 end

If the app is running under TheCampingServer, the above change can be saved and, when you refresh the app in your browser, the server will reload the app and update the database.

For now, Camping only uses the up method internally. The down method is only handy to you in your own scripting.

For more on how to script ActiveRecord to create and drop tables, see the section on ActiveRecord::Migration.

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