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Now that you can see the model represented in the browser, let s add some more fields to make it a little more interesting. Whenever you need to add or modify database fields, you should do so using a migration. In this case, let s add the excerpt and location fields to the articles table. You didn t need to generate the last migration (the one you used to create the articles table), because the model generator took care of that for you. This time around, you can use the migration generator. It works just like the model and controller generators, which you ve seen in action. All you need to do is give the migration generator a descriptive name for the transformation: $ rails generate migration add_excerpt_and_location_to_articles excerpt:string location:string invoke create active_record db/migrate/20100223232337_add_excerpt_and_location_to_articles.rb

>> => >> => >> => >> => >> => event = Event.find(:first) #<Event:0x268157c> category = Category.find_by_name('parties') #<Category:0x26388b8 @attributes={"name"=>"Parties", "id"=>"2"}> event.categories << category [#<Category:0x26388b8 @attributes={"name"=>"Parties", "id"=>"2"}>] event.categories.any true event.categories.size 1

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MySQL is the most common database system available from web hosting providers. This is because MySQL comes in an open source variety that s free to download and use. MySQL also has a reputation as being simple to use, and a large ecosystem has built up around it. MySQL support is given to Ruby with the MySQL library that s available as a RubyGem in generic Unix and Windows varieties. To install: gem install mysql The mysql gem provides a class and a number of methods to Ruby so that you can connect to a preinstalled MySQL server. It does not include the MySQL server itself! If you don t have a MySQL server to connect to, you ll need to ask for details from your web hosting provider or install a version of MySQL on your local machine.

Once the mysql gem is installed, connecting to and using a MySQL server from Ruby is almost as simple as using a SQLite database: require 'rubygems' require 'mysql' # Connect to a MySQL database 'test' on the local machine # using username of 'root' with no password. db = Mysql.connect('localhost', 'root', '', 'test') # Perform an arbitrary SQL query db.query("INSERT INTO people (name, age) VALUES('Chris', 25)") # Perform a query that returns data begin query = db.query('SELECT * FROM people') puts "There were #{query.num_rows} rows returned" query.each_hash do |h| puts h.inspect end rescue puts db.errno puts db.error end # Close the connection cleanly db.close This code demonstrates a basic, arbitrary SQL query, as well as a query that results in data being returned (in a row-by-row hash format). It also features basic error reporting by catching exceptions with a rescue block and using the error-retrieval methods provided by the MySQL library.

As you ve already seen, the generator creates a migration class in db/migrate prefixed by a number identifying when the migration was created. If you open the 20100223232337_add_excerpt_and_location_to_articles.rb file, you see the migration class with the code shown in Listing 3-4. As with the model generator, which prefilled the migration to some extent, passing field names and types as options to the migration generator prefills the generated class for you as long as you refer to the correct table name at the end of the migration name in this case, to_articles. Listing 3-4. The db/migrate/20100223232337_add_excerpt_and_location_to_articles.rb File class AddExcerptAndLocationToArticles < ActiveRecord::Migration def self.up add_column :articles, :excerpt, :string add_column :articles, :location, :string end def self.down remove_column :articles, :excerpt remove_column :articles, :location end end Looking at the add_column method, the first argument is the table name (articles), the second is the field name, and the third is the field type. Rails also fills in the self.down method to reverse the changes that this migration makes. Although it s unlikely at this point that you want to remove these fields, it s a

Here, we automatically associate a category with an event using the << operator. In this case, when you use << with has_and_belongs_to_many, it automatically saves the new association. Some things in Active Record don t happen until you say save, but this is one of the examples where that part is done automatically, We can even do this from the category s side of the association. Try the following:

note You can also access MySQL databases using the database-agnostic DBI library, covered later in

Like MySQL, PostgreSQL (pronounced post-gres-Q-L) is a free relational database server that s available under an open source license, allowing you to download and use it for free.

>> => >> => >> => category.events.empty false category.events.size 1 category.events.find(:first).title "Tiki Party"