Add Conditional Class Selectors in Haml as a rails til.

README.md

@@ -53,6 +53,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Attribute Getter without the Recursion](rails/attribute-getter-without-the-recursion.md)
 - [Attribute Was](rails/attribute-was.md)
 - [Capybara Page Status Code](rails/capybara-page-status-code.md)
+- [Conditional Class Selectors in Haml](rails/conditional-class-selectors-in-haml.md)
 - [Creating Records of Has_One Associations](rails/creating-records-of-has-one-associations.md)
 - [Show Pending Migrations](rails/show-pending-migrations.md)
 

rails/conditional-class-selectors-in-haml.md

@@ -0,0 +1,21 @@
+# Conditional Class Selectors in Haml
+
+You can assign a class selector to a tag in HAML like so:
+
+```ruby
+%div.active
+```
+
+You can conditionally assign a class selector in a concise manner like so:
+
+```ruby
+%div{ class: ( "active" if @thing.active? ) }
+```
+
+You can do multiple conditional class selectors with array syntax:
+
+```ruby
+%div{ class: [ ("active" if @thing.active?), ("highlight" if @thing.important?) ] }
+```
+
+[source](http://stackoverflow.com/questions/3453560/append-class-if-condition-is-true-in-haml-with-rails)