maybe do this through a migration?

db/migrate/8_add_primary_key_to_categorizations.rb

@@ -0,0 +1,12 @@
+class AddPrimaryKeyToCategorizations < ActiveRecord::Migration
+  def self.up
+    unless ::Categorization.column_names.include?("id")
+      add_column :blog_categories_blog_posts, :id, :primary_key
+    end
+  end
+
+  def self.down
+    remove_column :blog_categories_blog_posts, :id
+  end
+end
+