habtm and migrations

An article about using has_and_belongs_to_many with migrations. They say to:

create_table :table_name, :id => false do |table|
 ...
end

Here's a more fleshed out example:

    create_table :exercises_types, :id => false do |t|
      t.column "exercise_id", :integer
      t.column "type_id", :integer
    end