Remove duplication in Answer

* Removes smell Duplicated Code
* Applies refactoring Extract Method

example_app/app/models/answer.rb

@@ -7,11 +7,16 @@ class Answer < ActiveRecord::Base
   validates :text, presence: true
 
   def self.for_user(user)
-    joins(:completion).where(completions: { user_id: user.id }).last ||
-      NullAnswer.new
+    joins(:completion).where(completions: { user_id: user.id }).last_or_null
   end
 
   def self.most_recent
-    order(:created_at).last || NullAnswer.new
+    order(:created_at).last_or_null
+  end
+
+  private
+
+  def self.last_or_null
+    last || NullAnswer.new
   end
 end