Add Allow Associations To Be Optional as a Rails TIL

rails/allow-associations-to-be-optional.md

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+# Allow Associations To Be Optional
+
+As of Rails 5, whenever a `belongs_to` association is declared in an
+`ActiveRecord` model, it is assumed to be a required association.
+
+```ruby
+class Book < ApplicationRecord
+  belongs_to :author
+end
+```
+
+If we were to create a `Book` instance without an `Author`, then we would get
+an error – `Validation Failed: Author is missing`.
+
+We could either make sure to always create books with authors or, if it makes
+sense for our data model, we could treat the author as optional.
+
+The most explicit and precise way to make a relation optional is to declare it
+as such in the `belongs_to` directive.
+
+```ruby
+class Book < ApplicationRecord
+  belongs_to :author, optional: true
+end
+```
+
+Another approach, though I don't recommend it unless you have a strong reason,
+is to globally make associations optional. You can do this by adding the
+following line to your `config/application.rb` file.
+
+```ruby
+config.active_record.belongs_to_required_by_default = false
+```
+
+[source](https://www.bigbinary.com/blog/rails-5-makes-belong-to-association-required-by-default)