How to add reference column migration in Rails 6 with SQLite

Question

What is the correct way to add reference column migration in Rails 6 without getting SQLite3::SQLException: Cannot add a NOT NULL column with default value NULL ?

I can hack it to get it working; but, I am preparing a tutorial for a grad class, so I want to make sure I'm doing it "by the book".

The starting point is a Post class (think "blog post"). I want to add an Author class and set up an 1-to-many relationship between authors and posts. After adding the author class and running the corresponding migration, I then create a migration to add an Author reference to Post :

rails g migration AddAuthorToPost author:references

This command generates:

class AddAuthorToPost < ActiveRecord::Migration[6.0]
  def change
    add_reference :posts, :author, null: false, foreign_key: true
  end
end

The problem is, of course, that SQLite complains because it won't tolerate the potential for a null foreign key --- even if the Post table is empty: ( How to solve "Cannot add a NOT NULL column with default value NULL" in SQLite3? )

I looked back at the previous year's tutorial (prepared by a different instructor) and the generator did not add null: false to the migration. (See also Add a reference column migration in Rails 5 )

Removing null: false from the migration allows the migration to run; but, "disabling safety features" doesn't seem appropriate in a classroom setting :)

Is there a better way to do this?

Answer 1

By default, a foreign_key is required (on app level, not on database).

To disable, in config/application.rb add

config.active_record.belongs_to_required_by_default = false

Or

class YourModel < ApplicationRecord
  belongs_to :another_model, optional: true
end