问题
I understand that by default the id field is created and also: PRIMARY KEY (id).
How about the foreign key ?
I have Shops and Products tables and the following associations:
Shop: has_many :products
Product: belongs_to :shop
In Product I have also:
t.integer "shop_id"
which is meant to be the foreign key, and also:
add_index("products", "shop_id")
However, if I export the database I see only:
KEY `index_products_on_shop_id` (`shop_id`)
What should I do in order to add
FOREIGN KEY (`shop_id`) REFERENCES Shop(`id`)
?
回答1:
You can use the foreigner gem for adding foreign keys to your application. To get started add the following to your Gemfile
gem "foreigner"
After that you can easily add foreign keys in your migration like so:
add_foreign_key :products, :shops
This would add a foreign from product.shop_id to shop.id. See the documentation for more options like differently named keys or self-referencing tables.
来源：https://stackoverflow.com/questions/4406075/what-creates-the-foreign-key-constraint-in-ruby-on-rails-3