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.