Normally when do a database migration in Rails, when adding ownership from a model to another model, you use the concept of belongs_to or references:
create_table :comments do |t|
Interestingly enough, these methods are only available during the initial table creation. If you want to add a reference to a model that is created later, you have to do it the old fashioned way, by just adding a column:
add_column :comments, :group_id, :integer
Doing it this way is clean, easy, and definitely meets the KISS principle. But I do find it interesting that one can’t add an association later in the game. Sometimes the Rails way is just KISS and adding the column by hand.