Add Hash Slicing as a rails til.

README.md

@@ -95,6 +95,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Code Statistics For An Application](rails/code-statistics-for-an-application.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)
+- [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)
 - [Pretend Generations](rails/pretend-generations.md)
 - [Retrieve An Object If It Exists](rails/retrieve-an-object-if-it-exists.md)

rails/hash-slicing.md

@@ -0,0 +1,21 @@
+# Hash Slicing
+
+Rails' ActiveSupport adds
+[`#slice`](http://api.rubyonrails.org/classes/Hash.html#method-i-slice) and
+[`#slice!`](http://api.rubyonrails.org/classes/Hash.html#method-i-slice-21)
+to the `Hash` class. The interface of these two methods seems a little
+inconsistent though.
+
+```ruby
+> {a: 1, b: 2, c: 3}.slice(:a)
+=> {:a=>1}
+```
+
+The `#slice` method returns what is being sliced.
+
+```ruby
+> {a: 1, b: 2, c: 3}.slice!(:a)
+=> {:b=>2, :c=>3}
+```
+
+The `#slice!` method, on the other hand, returns what is being excluded.