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?

Note that in Rails 5 and in Rails 6 you may need to mark the corresponding association as optional if it's 1:n (belongs_to), as the default was changed:

belongs_to :author, optional: true

This is the corresponding Changeset.

To use the old behavior across your application, you can also set:

Rails.application.config.active_record.belongs_to_required_by_default = false

in config/initializers/new_framework_defaults.rb

The error you will usually see is:

ActiveRecord::RecordInvalid: Validation failed: Class must exist
    from /usr/local/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0.1/lib/active_record/validations.rb:78:in `raise_validation_error'

You may also need to update any migration: change null: false to true and run rake db:redo if it had already run.

Adding optional: true along with belongs_to :author in article model will do the job.

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.