Using the latest attributes in a Rails migration

Occasionally I need to make some change to a table via migrations, and then immediately use the attributes associated with the latest table inside the migration. Most of the time you can do this by defining the aspects of the class inside the migration - for example:

[sourcecode language='ruby']
class AddDepartmentToProducts < ActiveRecord::Migration

class Product < ActiveRecord::Base
belongs_to :department
end

def self.up
add_column :products, :department_id, :integer
# do something with product.department
end

def self.down
#...
end

end
[/sourcecode]

There's also another way that's not as neat, but gives you finer grained control, for example if you need to have different definitions of the class in the up and down migrations:

[sourcecode language='ruby']
class AddDepartmentToProducts < ActiveRecord::Migration

def self.up
add_column :products, :department_id, :integer
Object.class_eval <<-end_eval
class Apparel21::Product < ActiveRecord::Base
belongs_to :department
end
end_eval
# do something with product.department
end

def self.down
#...
end

end
[/sourcecode]