Add Creating Records of Has_One Associations as a rails til.

README.md

@@ -24,6 +24,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 ### rails
 
 - [Attribute Was](rails/attribute-was.md)
+- [Creating Records of Has_One Associations](rails/creating-records-of-has-one-associations.md)
 - [Show Pending Migrations](rails/show-pending-migrations.md)
 
 ### ruby

rails/creating-records-of-has-one-associations.md

@@ -0,0 +1,27 @@
+# Creating Records of Has_One Associations
+
+When working with a model, say a User, that has a `has_many` association
+with another model, say a Post, you can create a new post for a user like
+so:
+
+```ruby
+u1 = User.first
+=> #<User:0x...>
+u1.posts.create(title: "Some Title", content: "...")
+=> #<Post:0x...>
+```
+
+What about with a `has_one` association? Consider a Customer that has a
+`has_one` association with an Account. Rails provides this method for you:
+
+```ruby
+c1.create_account(account_number: 123, ...)
+=> #<Account:0x...>
+```
+
+Rails also gives you a similar `build` method:
+
+```ruby
+c1.build_account(account_number: 123, ...)
+=> #<Account:0x...>
+```