Rails 5 makes belongs_to association required by default

In Rails 5, whenever we define a belongs_to association, it is required to have the associated record present by default after this change.

It triggers validation error if associated record is not present.

class User < ApplicationRecord
end

class Post < ApplicationRecord
  belongs_to :user
end

post = Post.create(title: 'Hi')
=> <Post id: nil, title: "Hi", user_id: nil, created_at: nil, updated_at: nil>

post.errors.full_messages.to_sentence
=> "User must exist"

As we can see, we can't create any post record without having an associated user record.

How to achieve this behavior before Rails 5

In Rails 4.x world To add validation on belongs_to association, we need to add option required: true .

class User < ApplicationRecord
end

class Post < ApplicationRecord
  belongs_to :user, required: true
end

post = Post.create(title: 'Hi')
=> <Post id: nil, title: "Hi", user_id: nil, created_at: nil, updated_at: nil>

post.errors.full_messages.to_sentence
=> "User must exist"

By default, required option is set to false.

Opting out of this default behavior in Rails 5

We can pass optional: true to the belongs_to association which would remove this validation check.

class Post < ApplicationRecord
  belongs_to :user, optional: true
end

post = Post.create(title: 'Hi')
=> <Post id: 2, title: "Hi", user_id: nil>

But, what if we do not need this behavior anywhere in our entire application and not just a single model?

Opting out of this default behavior for the entire application

New Rails 5 application comes with an initializer named new_framework_defaults.rb.

When upgrading from older version of Rails to Rails 5, we can add this initializer by running bin/rails app:update task.

This initializer has config named Rails.application.config.active_record.belongs_to_required_by_default = true

For new Rails 5 application the value is set to true but for old applications, this is set to false by default.

We can turn off this behavior by keeping the value to false.

Rails.application.config.active_record.belongs_to_required_by_default = false

class Post < ApplicationRecord
  belongs_to :user
end

post = Post.create(title: 'Hi')
=> <Post id: 3, title: "Hi", user_id: nil, created_at: "2016-02-11 12:36:05", updated_at: "2016-02-11 12:36:05">