belongs_to associations are now validated unless 'optional: true'

[brentkearney]

After upgrading from Rails 5.0.7 to Rails 5.1.6, my Schedules model failed to validate when an associated belongs_to model is nil. The error is:

ActiveRecord::RecordInvalid:
       Validation failed: Lecture must exist

The Schedules model had:

belongs_to :lecture
validates_associated :lecture, allow_nil: true

After the upgrade, its necessary to add the optional option:

belongs_to :lecture, optional: true

Suggested addition to /guides/source/upgrading_ruby_on_rails.md:

Upgrading from Rails 5.0 to Rails 5.1

...

belongs_to associations are now automatically required: true

All model's belongs_to associations are now treated as if the required: true option is present. So if the association is optional, that must be declared, or validation will fail without the presence of the associated model record.

belongs_to :user, optional: true

[rafaelfranca]

Are you sure you are not seeing a deprecation message in Rails 5.1? This change was actually in Ruby 5.0.

[rafaelfranca]

And it is here in the upgrading guide https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#active-record-belongs-to-required-by-default-option

[bbozo]

Is there a way to keep the old behavior? It's a pain for legacy applications and I don't validate in the model

[everton]

Is there a way to keep the old behavior? It's a pain for legacy applications and I don't validate in the model

From the guides, as linked by @rafaelfranca we can see:

config.active_record.belongs_to_required_by_default = false

this gets back the original behavior.

[samssouza]

Something to note here also is that rails generators will also add null: false to your migration which will also prevent you from leaving association empty.