Ruby on Rails

Hello,

I'm new to Ruby/Rails and going through 'Beginning Rails 4'.  In chapter 6 it has me create a join table for an articles and categories table (articles_categories).  In the migrate file I've entered this code from the book:

class CreateArticlesCategories < ActiveRecord::Migration

  def change

    create_table :articles_categories, :id=> false do |t|

      t.references :article

      t.references :category

    end

  end

  def self.down

    drop_table :articles_categories

  end

end

My issue is that after I migrate this file, when I try to make an association between the article and category object (article.categories << category) it spits an error that article_id does not exist in articles_categories table.  It makes sense to me since the references above do no have _id appended in the class.  If I change the class to the following, creating the relationship between article and category works fine:

class CreateArticlesCategories < ActiveRecord::Migration

  def change

    create_table :articles_categories, :id=> false do |t|

      t.integer :article_id

      t.integer :category_id

    end

  end

  def self.down

    drop_table :articles_categories

  end

end

My question is, how can I get the 't.references' format to work so that AR looks for an 'articles' and 'categories' column, instead of the same with _id appended?