Add nullable foreign key in Rails

Referencing to Rails 4.2 add_foreign_key support:

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

How to create a nullable foreign key constraint, to allow the situation, where articles.author_id can be sometimes null?

There is nothing in the guide that implies add_foreign_key would make the corresponding foreign field "NOT NULL" or required. add_foreign_key simply adds a foreign key constraint whether the field is required or not (in your case author_id in articles).

Did you get an error when you tried this in your migration?

Here's the SQL that it would generate:

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

SO, if in your original migration of articles, author_id is null, then you can have foreign key that's nullable.