Add Attribute Was as a til for rails.

rails/attribute-was.md

@@ -0,0 +1,26 @@
+# Attribute Was
+
+When modifying the attributes of an `ActiveRecord` object, you may want to
+know what values the modified attributes used to have. `ActiveRecord` gets
+some handy methods from the `ActiveModel::Dirty` module that allow you to
+check these values out even if the object's attributes were changed before
+you received it (though you are out of luck once it has been saved).
+Just add `_was` onto the end of the attribute in question.
+
+```ruby
+>> pokemon.name
+=> "Charizard"
+>> pokemon.name = "Squirtle"
+=> "Squirtle"
+>> pokemon.name
+=> "Squirtle"
+>> pokemon.name_was
+=> "Charizard"
+>> pokemon.save
+   ...
+=> true
+>> pokemon.name_was == pokemon.name
+=> true
+```
+
+[source](http://api.rubyonrails.org/classes/ActiveModel/Dirty.html)