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The DataBase Interface (DBI) library is a database-agnostic library that uses database drivers and provides a generic interface to them. This allows you to write your code without a specific database in mind, and lets you use the same methods on nearly all databases. This means you lose the ability to access some of the more advanced features offered by database drivers, but DBI can give you a little more simplicity and cross-database operability. The DBI approach is now considered somewhat obsolete, however, so this section is provided merely to give you a pointer in case you need to access the features it provides. DBI is available as a RubyGem called, simply, dbi. Alternatively, you can refer to the instructions and files available at http://ruby-dbi.rubyforge.org/. At the time of writing, DBI was last updated in late 2008, so it appears to still be under active development.

Figure 3-5. Error messages for an article If you ve done any web development before, you know that validating fields is a major nuisance. Thankfully, Rails makes it easy.

has_many :registrations has_many :attendees, :through => :registrations, :source => :user has_and_belongs_to_many :categories end

note If you re a Perl programmer, DBI will be instantly familiar to you, as it follows many of the same

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Once you have Ruby DBI installed, along with any database drivers you require, DBI makes it easy to use a database supported by your drivers, as demonstrated in this example: require 'dbi' # Connect to a database db = DBI.connect('DBI:Mysql:db_name', 'username', 'password') # Perform raw SQL statements with 'do', supports interpolation db.do("INSERT INTO people (name, age) VALUES ( , )", name, age) # Construct and execute a query that will return data in # the traditional way query = db.prepare('SELECT * FROM people') query.execute while row = query.fetch do puts row.inspect end query.finish # Pull data direct from the database in a single sweep # This technique is cleaner than the previous db.select_all('SELECT * FROM people') do |row| puts row.inspect end db.disconnect In this example, you connect to a MySQL database using the Mysql driver, but if you had the Oracle driver installed for use by DBI, you could just as easily connect to an Oracle database using a DSN such as DBI:OCI8:db_name. For more up-to-date information about this functionality, and how to get other drivers, refer to http://ruby-dbi.rubyforge.org/.

class User has_many has_many has_many end < ActiveRecord::Base :events :registrations :activities, :through => :registrations, :source => :event

NOTE Notice that you don t need to restart the web server when you make changes to your project files in the

So far in this chapter, you ve worked directly with databases using a whole new language: SQL. Working with a database in this way is more efficient and reliable than putting data into text files, as you did earlier, but ActiveRecord makes it easier still. ActiveRecord is a product of the Ruby on Rails framework, which we ll be covering in 13, but can be used entirely independently of it. ActiveRecord will be covered in more depth in that chapter, but deserves a brief summary here. ActiveRecord abstracts away all the details of SQL and database design, and makes it possible to relate to items within databases in an object-oriented fashion, as you did with PStore.

You ll notice that we reworked how we are naming associations. One aspect of the Rails philosophy is that we should always be questioning and refactoring our code to work with best practices. So, in this incarnation, events that we are attending as a user are called activities, and events are events that we are hosting. So, as an added benefit, has_many :through allows us to easily have nice names on our associations. The :source option lets us define the source name of the association. In this case, the activity in question is an event, so we set the :source option accordingly. The through associations let us pretend that the middle table (registrations) doesn t exist. Let s play with this on the console to see how it works. We ll start by finding any event and creating a brand-new user that can register for the event:

Web application Frameworks: Rails, Sinatra, and Ramaze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349