Add Retrieve An Object If It Exists as a rails til.

rails/retrieve-an-object-if-it-exists.md

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+# Retrieve An Object If It Exists
+
+Rails' Active Support provides the `blank?` and `present?` convenience
+methods as extensions to many objects. It also extends the Object class by
+providing the [`presence`](http://api.rubyonrails.org/classes/Object.html#method-i-presence)
+method. This method returns the *receiver* if it is not blank, otherwise
+it returns nil.
+
+Instead of doing
+
+```ruby
+User.nickname.present? ? User.nickname : User.firstname
+```
+
+I can simply do
+
+```ruby
+User.nickname.presence || User.firstname
+```