rails 3: foreign key in migration

class ExampleMigration < ActiveRecord::Migration def self.up

create_table :products do |t|
t.references :category
end
#add a foreign key
execute <<-SQL ALTER TABLE products ADD CONSTRAINT fk_products_categories FOREIGN KEY (category_id) REFERENCES categories(id) SQL add_column :users, :home_page_url, :string rename_column :users, :email, :email_address

end

def self.down

rename_column :users, :email_address, :email
remove_column :users, :home_page_url
execute "ALTER TABLE products DROP FOREIGN KEY fk_products_categories"
drop_table :products

end