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good idea to maintain reversibility. The remove_column method is the opposite of add_column. The only difference in its arguments is that you don t need to specify the field type. With this new migration in place, use the Rake task to apply it and make the changes to the database: $ rake db:migrate == AddExcerptAndLocationToArticles: migrating ================================ -- add_column(:articles, : excerpt, :string) -> 0.0236s -- add_column(:articles, : location, :text) -> 0.0013s == AddExcerptAndLocationToArticles: migrated (0.0164s) ========================= If all goes according to plan, the articles table has two new fields. You could edit the view templates in the app/views/articles folder to add form elements for the new fields, but instead let s call the generator again (you learn about views in 6): $ rails generate scaffold Article title:string location:string excerpt:string body:text published_at:datetime --skip-migration Press Y when asked if you want to overwrite some files, and you re finished, as you can see in Figure 3-4.

Although PostgreSQL offers many of the same features as MySQL, it s quite different. PostgreSQL users claim it s faster and more stable and offers more features. It is also often claimed that PostgreSQL follows SQL standards more correctly, whereas MySQL is more pragmatic (in the sense that it s more willing to break away from established standards) and extends the SQL language for its own benefits. As with MySQL, PostgreSQL access is achieved in Ruby with a library, also available as a RubyGem. To install, use this code: gem install postgres On some setups, it might be necessary to specify where PostgreSQL is installed, like so: gem install postgres -- --with-pgsql-dir=/var/pgsql The interface for the postgres library is totally different from that for MySQL or SQLite, and it s documented in full at http://ruby.scripting.ca/postgres/rdoc/. (You can also access PostgreSQL databases using the database-agnostic DBI library, covered in the later section DBI: A Generic Database Connectivity Library. )

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We just did the opposite of the previous test. We said that has_and_belongs_to_many works in both directions, right So, we simply found our new category and asked it for its first event, which we can see is now Tiki Party, because that s what we associated in the other direction, too. Using has_and_belongs_to_many is a very simple way to approach many-to-many associations. However, it has its limitations. Before you re tempted to use it for associating users with events they would like to attend (activities), we should point out that it has no way of storing additional information on the join. What if we wanted to know when someone decided to attend an event, or how many people the attendee is going to bring along This kind of data fits naturally in the join table. Rails includes another type of association called has_many :through, which allows us to create rich joins like this.

note There s also a pure Ruby equivalent of the postgres library called postgres-pr. If the postgres gem

Figure 3-4. Additional fields added to the new article form This exposes one of the issues of this type of scaffolding: when you generate new versions of the scaffold files, you run the risk of overwriting custom changes you may have made.

Oracle Corporation is a company that provides database tools, services, and software, and is primarily known for the development of the Oracle RDBMS, a relational database management system. The Oracle database system is not open source and is not, in its full version, available for free to use commercially. However, it s reasonably popular in enterprise situations, so you might need to know how to connect to it.

Sometimes when you re modeling a many-to-many association, you need to put additional data on the join model. But, since Active Record s has_and_belongs_to_many uses a join table (for which there is no associated model), there s no model on which to operate. For this type of situation, you can create rich many-to-many associations using has_many :through. This is really a combination of techniques that end up performing a similar but more robust version of has_and_belongs_to_many. The basic idea is that you build a full model to represent the join table. In our application, we will build a model called a Registration. You could say that users have many registrations that belong to an event, or users have many events through registrations. Figure 5-5 illustrates this relationship.