Referential integrity, it gives me that warm and fuzzy feeling inside. That’s why I was so excited when Rails added support for managing foreign keys with ActiveRecord. However, I got a little confused when implementing a foreign key on a Model that had an internal tree structure with self-referential parent/child associations. Here’s the final implementation.
Take the following Model with a self-referential parent / child association:
class Category < ActiveRecord::BaseActiveRecord::Base belongs_to :parent, class: 'Category' has_many :children, class: 'Category', foreign_key: 'parent_id' end
To add a foreign key for that association we would put the following in our migration:
add_reference :categories, :parent, index: true add_foreign_key :categories, :categories, column: 'parent_id'
This will add a parent_id column to the categories table, index that column and specify the foreign key. This will ensure we don’t have any Category that references a non-existent parent.
