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Migrations are
designed to be cross-platform, so you can usually run the same migration across multiple
databases. (Of course, if you use any database-specific features, the migration won??™t be
cross-platform.) Migrations are versioned, so you can upgrade and downgrade them as
you see fit. Rails also keeps track of the current version of your database, so if you have a
number of migrations, Rails will run only the new migrations.
Create a new migration by using the following command:
ruby script/generate migration initial_schema
CHAPTER 5 n CONNECTING YOUR REPORTS TO THE WORLD 78
create db/migrate
create db/migrate/001_initial_schema.rb
This creates a skeleton migration. The initial db/migrate/001_initial_schema.rb file
looks like this:
class InitialSchema < ActiveRecord::Migration
def self.up
end
def self.down
end
end
As you can see, it??™s a single class that inherits from ActiveRecord::Migration, and it
has two class methods: up and down, which upgrade and downgrade the version of the
database structure, respectively, when the migration is run.


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