Add Autosave False On ActiveRecord Associations as a rails til.

README.md

@@ -102,6 +102,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [All or Nothing Database Transactions](rails/all-or-nothing-database-transactions.md)
 - [Attribute Getter without the Recursion](rails/attribute-getter-without-the-recursion.md)
 - [Attribute Was](rails/attribute-was.md)
+- [Autosave False On ActiveRecord Associations](rails/autosave-false-on-activerecord-associations.md)
 - [Capybara Page Status Code](rails/capybara-page-status-code.md)
 - [Code Statistics For An Application](rails/code-statistics-for-an-application.md)
 - [Conditional Class Selectors in Haml](rails/conditional-class-selectors-in-haml.md)

rails/autosave-false-on-activerecord-associations.md

@@ -0,0 +1,44 @@
+# Autosave False On ActiveRecord Associations
+
+A relationship between two ActiveRecord models can be established with a
+`has_one` or `has_many` association. This relationship has some
+implications. By default, saving a record will also save the associated
+records that have since been built. Consider this example of users that have
+many posts (`has_many posts`).
+
+```ruby
+> u = User.first
+#=> #<User ...>
+> u.posts
+#=> []
+> u.posts.build(title: "Some Title", content: "This is a post")
+#=> #<Post ...>
+> u.save
+#=> true
+> u.posts(reload: true)
+#=> [#<Post ...>]
+```
+
+When the user is saved, the associated post that was built for that user
+also gets saved to the database.
+
+If the association is instead defined with the `autosave` option set to
+false, then saving a record will not cause associated records to also be
+saved. The associated records will need to be saved explicitly. Consider the
+same example from above, but with `has_many posts, autosave: false`.
+
+```ruby
+> u = User.first
+#=> #<User ...>
+> u.posts
+#=> []
+> u.posts.build(title: "Some Title", content: "This is a post")
+#=> #<Post ...>
+> u.save
+#=> true
+> u.posts(reload: true)
+#=> []
+```
+
+The post wasn't saved with the user and it wasn't saved explicitly, so it
+isn't persisted to the database.