Connecting Rails to legacy databases isn’t actually that hard – depending on the database you start out with. Recently, we needed to perform some statistical analysis on a large Movable Type database. Rather than wrestling with endless SQL queries at the prompt, it made sense to abstract out a little and build some kind of modelled front end to the statistics.

The most obvious tool for me to use was Rails; I’m familiar with it, I like the way ActiveRecord works, and it means that I can poke around the database from script/console if I need to.

The reason this turned out not to be too hard is because whilst Movable Type doesn’t conform to Rails’ opinionated ideas of what a schema should look like, it is still a well-designed and normalised database. Because of this, we can teach ActiveRecord to understand the database.

First of all, we start by creating our models: for our needs, Blog, Comment, Post and Author. We generate them in the usual manner – script/generate model blog. Once we’ve done that, we delete the migration files in db/migrate it creates, because we’re not going to use them.

Next, we point config/database.yml to the Movable Type database.

And then, we build our relationships thus:

class Blog < ActiveRecord::Base
  set_table_name "mt_blog"
  set_primary_key "blog_id"

  has_many :entries, :foreign_key => "entry_blog_id", :order => "entry_created_on"

class Entry < ActiveRecord::Base
  set_table_name "mt_entry"
  set_primary_key "entry_id"

  has_many :comments, :foreign_key => "comment_entry_id"
  belongs_to :blog, :foreign_key => "entry_blog_id"
  belongs_to :author, :foreign_key => "entry_author_id"

class Comment < ActiveRecord::Base
  set_table_name "mt_comment"
  set_primary_key "comment_id"

  belongs_to :entry, :foreign_key => "comment_entry_id"

class Author < ActiveRecord::Base
  set_table_name "mt_author"
  set_primary_key "author_id"

  has_many :entries, :foreign_key => "entry_author_id"

The set_table_name method tells the ActiveRecord class what table to look at, and the set_primary_key method does exactly what it says on the tin. (It also makes sure that #to_param works correctly based on whatever our new primary key is, which is handy). Beyond that, we simply have to specify the foreign keys on our relationships and everything plays ball; we can now access blog.entries just as we do with a typical Rails setup. It’s now easy to write the rest of our Rails app – model methods, controllers, views – just as we normally would. We’re just using the MT database to pull out our data.

And if you’re wondering: yes, it made the manipulation a lot easier, and a few hours poking at the console began to yield some interesting algorithms to apply.

1 comment on this entry.

  • Ramesh Krishnan | 21 Feb 2009

    Many thanks. Helped me immensely.