Add Allow Associations To Be Optional as a Rails TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1300 TILs and counting..._
+_1301 TILs and counting..._
 
 ---
 
@@ -763,6 +763,7 @@ _1300 TILs and counting..._
 - [Access Secrets In A Rails 5.2 App](rails/access-secrets-in-a-rails-5-2-app.md)
 - [ActiveRecord Query For This Or That](rails/active-record-query-for-this-or-that.md)
 - [Advance The Date](rails/advance-the-date.md)
+- [Allow Associations To Be Optional](rails/allow-associations-to-be-optional.md)
 - [Allow List Params Anywhere With Strong Params](rails/allow-list-params-anywhere-with-strong-params.md)
 - [All or Nothing Database Transactions](rails/all-or-nothing-database-transactions.md)
 - [Alphabetize Schema Columns To Keep Them Consistent](rails/alphabetize-schema-columns-to-keep-them-consistent.md)

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

@@ -0,0 +1,35 @@
+# 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)