You just add the belongs_to and the has_many associations to the model which you have to do anyway regardless of the method used to generate in the first place so it makes no difference.
If however you forgot to add the foreign key to the belongs_to model then you would have to either create a new migration to add that column to the database table or migrate your database back one version and add the column in to the create migration as described by jmesserer in post #2
Which way you choose depends totally on you.
If you have data in that table that you would loose by migrating back, manually editing the migration file then migrating normally again then you would be better off to create a new migration to add the column.
It sounds far more complex than it really is. Once you get used to these scenarios you will find that it is actually really really simple especially when compared to other languages.
You need to get used to editing the migration files anyway so you can set up indexes, specify default values for columns and provide constraints
class CreatePlugins < ActiveRecord::Migration
create_table :plugins do |t|
t.boolean :available, :default => false
t.decimal :price, :precision => 8, :scale => 2, :default => 0
t.string :plugin_type, :limit => 10
Note the available, price and plugin_type entries have been manually edited prior to running rake db:migrate to provide
1) a 10 character limit on the size of the plugin_type column
2) Default the price to a value of 0 and set the column to be a maximum of 8 digits with 2 decimal places
3) Set the available column to default to a value of false
There are many more things you need to know but just take it slowly, experiment, read some books (AWDWR), check out the api docs and have some fun.
Hope that makes things clearer for you.
What you want and what you need are too often not the same thing!
When your head is hurting from trying to solve a problem, stop standing on it. When you are the right way up you will see the problem differently and you just might find the solution.
(Quote by me 15th July 2009)