Rails 4.2 foreign key

Question

Rails 4.2 newly supports adding and removing foreign keys (in migrations), like:

# add a foreign key to `articles.author_id` referencing `authors.id`
add_foreign_key :articles, :authors

What I don't understand is: How is this

add_foreign_key :articles, :authors

different from this:

add_column :articles, :author_id, :integer

Thank you for any clarification!

score 9 · Accepted Answer · answered Nov 29 '14 at 10:23

9

The difference is that line:

add_foreign_key :articles, :authors

will actually generates this:

ALTER TABLE "articles" ADD CONSTRAINT articles_author_id_fk FOREIGN KEY ("author_id") REFERENCES "authors" ("id");

While this:

add_column :articles, :author_id, :integer

will generate:

ALTER TABLE "articles" ADD COLUMN author_id INT(11);

Both are different because add_foreign_key will add just a foreign key constraint, while add_column adds a column not a constraint.

answered Nov 29 '14 at 10:23

Surya

Thanks. So is it correct to conclude that using ```add_foreign_key``` will result in better database performance (because it adds a foreign key)? – TomDogg Nov 29 '14 at 10:33
1

technically, it *decreases* `database performance` as it forces the DB to check this constraint for every update/insert. But it adds `referential integrity` i.e. ensures your data "makes sense", and you don't get inconsistent/orphaned data – Don Cheadle Feb 05 '15 at 17:10

1 Answers1