5.0.0.beta2 version is released. One of the changes I noticed is belongs_to associations by default required is true, that means if you create an object by setting foreign key to be nil it will throw a must exist validation error. This change will have some impact if you are migrating an existing rails 4 application in to rails 5 and foreign key is not mandatory in your application.
For example, if your User model has city_id as forign key, then if you create user without adding city_id then rails 5 will throw validation error.
You can turn off this in entire application by setting Rails.application. config.active_record.belongs_to_required_by_default = false in config/initializers/active_record_belongs_to_required_by_default.rb file.
# Be sure to restart your server when you modify this file.# Require `belongs_to` associations by default. This is a new Rails 5.0# default, so it is introduced as a configuration option to ensure that apps# made on earlier versions of Rails are not affected when upgrading.Rails.application.config.active_record.belongs_to_required_by_default=false